JPH01144821A - Analogue/digital converter - Google Patents

Abstract

PURPOSE:To prevent the increase of a quantization error by respectively adjusting analogue reference voltage impressed on many comparators for executing A/D conversion and obtaining an optional conversion characteristic. CONSTITUTION:Voltage holding means for holding the reference voltage of the analogue comparator are provided on the respective analogue comparators of a comparator unit CMP and time constant circuit means such as an RC circuit are provided on the respective voltage holding means so as to set the reference voltage. That means, when prescribed DC voltage is impressed on the time constant circuit, the output of that circuit is changed with the lapse of time. For example, the analogue voltage of an exponential function curve can be obtained. When the analogue voltage is sampled at an optional time, the voltage of an optional point on the curve can be obtained. Therefore, the voltage set for the reference voltage can be controlled by time control. Thus, by the time corresponding to the respective reference voltage is calculated by using a computer such as a microcomputer CPU and controlling the set timing of the respective reference voltage according to that calculation result, the whole reference voltage can be automatically set.

Description

[Detailed description of the invention] [Field of invention] The present invention relates to an analog/digital conversion device.

In particular, the present invention relates to an analog/digital conversion device that can be set to nonlinear conversion characteristics.

[Prior art] For example, when reading one image information and displaying the result 1
A CCD image sensor is often used as an image input device, and a CRT (cathode ray tube) display device is often used as an image display device. By the way, the imaging characteristic of a CCD image sensor is linear, but the relationship between the input signal level and brightness of a CRT display device is exponential. Therefore, in order to correlate the density of the input image with the brightness of the displayed image, an inverse function of the exponential function must be used between the image sensor and the CRT display to cancel the influence of the exponential characteristic of the CRT display. It is necessary to provide a correction circuit with certain logarithmic characteristics. This type of correction circuit is generally called a γ compensation circuit.

Conventionally, in digital image processing circuits, input levels are determined by using a memory table such as a ROM in which information about the output level to be converted is written in advance in all addresses of a memory in which one address is assigned to each input level. A gamma correction method that performs nonlinear conversion between the output level and the output level is generally used.

However, this type of method has the disadvantage that quantization errors increase because the conversion characteristics are curved. For example, when γ-correcting 8-bit image information, the fourth
As is clear from the conversion characteristics shown in the figure, in areas where the input level is small, the step interval of the output level is large, and in areas where the output level is large, the step interval of the input level is large, so all 256 gradations are Not used effectively. In reality, when this type of conversion is performed, when the number of input gradations is 256, the number of output gradations is reduced to 140 degrees, and the quantization error increases considerably.

In addition, other correction methods are disclosed in Japanese Patent Application Laid-Open No. 62-183678, for example.
It is disclosed in No. 183679 and Jikai No. 62-183679. These devices have a correction function added to the Δ/D converter that converts analog signals into digital signals. That is, by connecting a resistor externally in parallel with a part of the ladder resistance inside the A/D converter, the A/D conversion characteristics are set to nonlinear characteristics approximating a broken line, and γ correction is performed thereby. It is.

Let me explain it a little more clearly. The internal depletion of the 8-bit A/D converter is as shown in FIG. 5a.

That is, the terminals include an analog input terminal Vin, digital output terminals (B1 to B8), a clock input terminal (CLK) that determines conversion timing, a chip enable terminal, and reference voltage terminals (REF+, REF-).

R/4. R/2, 3R/4), etc.

Inside, there are 256 comparators, latches that hold the output information of the comparators, an encoder that converts the output of the latches into 8-bit binary code, and flip-flops. An analog input voltage is commonly applied to one input terminal of each of the comparators. The other input terminal of the comparator is connected to the fundamental voltage terminal R[E
256 types of reference voltages equally divided by 256 ladder resistors connected between F+ and Ill[EF- are applied.

Reference voltage terminal R/4. R/2 and 3R/4 are terminals RE
They are connected to positions that equally divide the 256 ladder resistors connected between F+ and REF- into four.

The input/output characteristics of an A/D converter are normally linear, but the input/output characteristics of the A/D converter are linear. By connecting external resistors to R/2 and 3R/4, it is possible to change the nonlinear characteristics to a polygonal line approximation. For example, as shown in FIG. 5b, terminal R [EF- and terminal R
If you connect an external resistor with the same resistance value as the combined resistance value of the ladder resistance of that part between terminal R/2,
The potential of 2 is 1/2 between the terminals REF+ and rl[iF-.
It becomes 4 potential. Therefore, for the full scale of A/D conversion, output values 0 to 127 are assigned to the lower level 1/4, and output values 128 to 2 are assigned to the upper 3/4.
55 is assigned.

In other words, the input/output characteristic becomes a broken line as shown in FIG. 5C, and the slope of the characteristic changes at a level of 1/4 of the full scale. By using other terminals, it is possible to obtain even more multi-stage broken lines, and it is possible to realize characteristics that are close to the required Mo11 positive characteristics.

However, this type of correction is only an approximation, and highly accurate γ correction cannot be expected.

[Object of the Invention] The first object of the present invention is to provide an analog/digital converter having highly accurate nonlinear input/output characteristics, which can be used for gamma correction, etc., and which has a relatively small quantization error. A second object of the present invention is to provide an analog/digital conversion device in which setting of characteristics can be easily changed to a target curve.

[Structure of the Invention] In order to achieve the above object, in the present invention, at least one voltage adjustment means for adjusting each of the reference voltages applied to the analog comparators is associated with each analog comparator. establish.

In this way, the reference voltage of each analog comparator can be adjusted arbitrarily. If each of these reference voltages is set according to a predetermined nonlinear curve, an accurate correction curve that is not a polygonal line approximation can be obtained. In this case, the quantization error does not increase due to the change in characteristics, and for example, in the case of 8-bit A/D conversion, the number of possible gradations of the output data is 256.

The voltage adjustment means can be realized, for example, by including a variable resistor in each resistor of the ladder resistor that determines the reference voltage. However, in that case, 8-bit A/D
The converter requires 256 variable resistors, and in order to adjust the characteristics, all 256 variable resistors must be adjusted, which requires an enormous amount of effort and time.

Therefore, in a preferred embodiment of the present invention, the reference voltage of each analog comparator is configured to be automatically set on the electric circuit side. Therefore, each analog comparator is provided with a voltage holding means for holding its reference voltage, and in order to set the reference voltage in each voltage holding means,
A time constant circuit means such as an RC circuit is provided. That is, if a predetermined DC voltage is applied to a time constant circuit, an analog voltage that changes over time, for example, an exponential curve, is obtained as the output of the circuit. If you sample the voltage at any time, you can obtain the voltage at any point on the curve.

In other words, the voltage set as the reference voltage can be managed by time control. Therefore, by calculating the time corresponding to each reference voltage using a calculation device such as a microcomputer and controlling the setting timing of each reference voltage according to the result, all reference voltages can be automatically set. sell.

Other objects and features of the present invention will become apparent from the following description of embodiments with reference to the drawings.

[Embodiment] FIG. 1 shows the configuration of one type of A/D conversion device that implements the present invention. Referring to FIG. 1, this A/D converter includes analog switch units ASI, AS2. Integrating circuit unit RCC, comparator unit CMP, latch LT, encoder ENC, flip-flop FF,
Three-state buffer and microcomputer CP
It is equipped with U and is configured to provide 8-bit digital output.

In FIG. 1, a latch LT and an encoder ENC.

Flip-flop FF and three-state buffer are
It has a configuration similar to that of conventional A/D converters and operates in the same manner.

In order to obtain an 8-bit digital output, the comparator unit CMP is equipped with 256 analog comparators. One input terminal of the analog comparators is all commonly connected to the analog signal input terminal Ain of the A/D converter. Different reference voltages are applied to the other input terminals of the analog comparators.

Based on this, the integral circuit unit RCC outputs 14 m pressure. The integrator circuit unit RCC has two independent R C integrator circuits each consisting of one resistor and one capacitor.
It is equipped with 56 sets, and the output terminal of each RC integration circuit is
It is connected to the input terminal of each analog comparator of the comparator unit CMP.

One analog switch unit ASI includes two analog switches, and the other analog switch unit AS2 includes 256 analog switches. One end (input side) of the 256 analog switches of the analog switch unit AS2 are all commonly connected to the output terminal of the analog switch unit ASI, and
The other end (output side) of each analog switch of S2 is connected to each input terminal of the integrating circuit unit RCC.

On/off of each analog switch in the analog switch unit AS2 is controlled by the microcomputer CPU. The analog switch unit ASI sets the level applied to the input terminal of the analog switch unit AS2 to either a predetermined reference potential Vref or the ground potential in response to a control signal (binary signal) from the microcomputer CPU.

Next, the basic principle when performing processing such as γ correction in this A/D conversion device will be explained. In order to perform the γ correction, for example, as shown in FIG. 4, the relationship between input and output may be made non-linear. However, when a digital signal is processed and γ correction is performed, an increase in quantization error is unavoidable. Therefore, in this embodiment, by adjusting the reference voltage (i.e., threshold level) of each of the 256 analog comparators, nonlinear characteristics can be obtained when converting an analog signal into a digital signal. It is composed of: Therefore, the quantization error of this A/D converter including γ correction processing is 1 similar to that of a normal A/D converter, and γ
The correction does not cause any particular deterioration.

The reference voltage applied to each analog comparator is automatically set by the microcomputer CPU. The integrating circuit unit RCC has the functions of changing the reference voltage and maintaining the set base IP and voltage. That is, when a DC voltage is applied to each integrating circuit, a voltage that changes over an hour and changes exponentially is obtained at its output terminal.

In other words, when the capacitor of each integrating circuit is completely discharged, the analog switch unit ASI, A
When applying a constant voltage V ref via S2, and assuming that the capacitance of the capacitor is C5 and the resistance value is R, the output voltage V of the integrating circuit is expressed by the following equation (1).

V=Vref (1-e-t) (1) That is, the voltage V output from the integrating circuit increases with time as shown in FIG. However, unit A
When the analog switch S2 is turned off, the input impedance of the analog comparator connected to the output terminal of the integrating circuit is very large, so the charge stored in the capacitor of the integrating circuit is hardly discharged. Therefore, the base voltage applied to the analog comparator is held at the voltage output by the integrating circuit when the analog switch of unit AS2 is turned off.

Therefore, as shown in FIG. 2, each analog switch S Wa , S W b , S
Wc.

SWd and SWe* "" are respectively time Ta.

Turn on only Tb, Tc, Td, Te,...

When a constant voltage V ref is applied to the integrating circuit, different voltages Va, Vb, Vc, Vd, Ve, ``'' are held at the output of the integrating circuit to which each analog switch is connected. Therefore, by controlling the time during which the analog switch is turned on, the reference voltage of each analog comparator can be set individually.

This time is controlled by the microcomputer CPU.

Threshold data for γ correction is externally applied to the microcomputer CPU.

That is, 256 threshold values defining each of the 256 gradation levels are applied to the microcomputer CPU as digital data. This threshold data DVO~
D V255 is the threshold value table M1 shown in Fig. 3a.
is stored in

An outline of the operation of the microcomputer CPU is shown in FIG. 3b. This will be explained with reference to FIG. 3b.

First, threshold data input from the outside is stored in the table M1 area of its internal memory (RAM).

Next, based on the above-mentioned equation (1) showing the relationship between voltage and time, each threshold value on table Ml is set to time data TO
~T255 and store them in the area of table M2.

Next, the reference voltage held in the integrating circuit RCC is reset. That is, in order to discharge the charge in the capacitor, all the switches of the analog switch unit AS2 are turned on, and the analog switch unit ASI is controlled so that the AS
Set input terminal 2 to ground potential. As a result, the output voltage of the integrating circuit RCC becomes 0 within a predetermined time.6 When the output voltage becomes 0, in order to release the reset state, the analog switch unit ASI is controlled and the input terminal of the unit AS2 is , a constant voltage Vref is applied.

Next, the counter register CN is cleared to 0, and all switches 5W(O) to 5W of the analog switch unit AS2 are cleared.
5W (255) is set to ON and the internal timer TM is started.

Then, the count value of the timer TM is monitored. That is.

The time data indicated by count register CN on table M2 is compared with the count value of TM.

Wait until TM≧M 2 (CN), then turn off the analog switch 5W (CN) indicated by the count register CN, increment the count register CN, and continue this process until CN exceeds 255. repeat.

As a result, the 256 analog switches are sequentially turned off, and each time the output voltage of one integrating circuit is determined (fixed). This voltage is applied to the analog comparator as a reference voltage.

Therefore, by changing the threshold data DVO to DV255 applied to the microcomputer CPU, the conversion characteristics (input/output characteristics) of this A/D conversion device can be changed arbitrarily.

Incidentally, in the above embodiment, a filter integrator circuit composed of C and R was used as the one time constant circuit, but if the characteristic that the output level changes with time can be obtained similarly,
Other circuits may also be used.

Furthermore, as is clear from equation (1) above, the output voltage V of the integrating circuit changes not only with one hour but also with the time constant (CR) and input voltage (Vref), so these parameters should also be changed. The circuit configuration may be such that the reference voltage applied to each analog comparator is set using the reference voltage applied to each analog comparator.

Furthermore, in the above embodiment, 256 independent integrating circuits are provided at the input terminals of all 256 analog comparators, and a constant voltage (Vref) is provided at their common input terminal.
), but it may be changed to the following configuration, that is, by using only one time constant circuit (for example, an integrating circuit) and applying a constant voltage Vref to its input terminal, 256 sample-and-hold circuits may be connected to the output terminal, and the voltage obtained at each output terminal of the sample-and-hold circuit may be applied to each analog comparator as a base Q voltage.

FIG. 6 shows an embodiment of an A/D converter having the simplest configuration. Referring to FIG. 6, in this embodiment, each resistance element constituting the ladder resistor RX connected between the reference voltage terminals +REF and -REEF is composed of a variable resistor. Therefore, by adjusting the resistance value of each variable resistor, the voltage division ratio of the ladder resistor changes, and the voltage applied to the analog comparator connected to it changes.These 256 variable resistors By adjusting A/
It is also possible to set the characteristics of the D conversion device to predetermined nonlinear characteristics, and it can also be used as a γ correction circuit.

However, in this embodiment, the variable resistor must be adjusted by a human, and changing the setting to a predetermined conversion characteristic requires time and effort.

[Effects] As described above, according to the present invention, arbitrary conversion characteristics are obtained by adjusting the analog base Kl pressure applied to a large number of comparators for A/D conversion, respectively.
No increase in quantization error occurs. Furthermore, since all of the quantization levels can be adjusted individually, accurate correction characteristics that are not approximate can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the electrical circuit configuration of one type of A/D conversion device implementing the present invention. FIG. 2 is a timing chart showing the reference voltage setting operation in the device shown in FIG. FIG. 3a is a memory map showing a table on the microcomputer CPU shown in FIG. 1, and FIG. 3b is a flowchart showing an outline of the operation of the CPU. FIG. 4 is a graph showing an example of γ correction characteristics by digital processing. Fig. 5a is a plot diagram showing a conventional example of an A/D converter, Fig. 5b is a block diagram showing a configuration example of an A/D converter that obtains a polygonal conversion characteristic, and Fig. 5c is a diagram of the circuit of Fig. 5b. It is a graph showing input/output characteristics. FIG. 6 is a block diagram showing another embodiment of the present invention. ASI: Analog switch unit AS2: Analog switch unit RC: C: Integrating circuit unit (voltage adjustment means) CMP:
Comparator unit LT: Latch ENC: Encoder FF: Flip-flop CPU: Microcomputer RX: Ladder resistor (voltage adjustment means) Math 3a Zubo 3b Zushin 41 Father transfer 5c Zuno - Human power KEF4-

Claims (4)

[Claims] (1) It has multiple analog comparators, applies an analog input voltage to one input terminal of each analog comparator, applies different reference voltages to the other input terminal, and outputs multiple analog comparators. In an analog/digital converter that encodes a comparison result into a multi-bit signal and outputs it: Voltage adjustment means for adjusting each of the reference voltages applied to the analog comparators is associated with each analog comparator. , an analog/digital conversion device comprising at least one of each. (2) The analog/digital converter according to claim 1, wherein the voltage adjustment means is a variable resistor. (3) The voltage adjusting means includes a voltage holding means provided for each analog comparator, a voltage generating means, and a time constant inserted between the voltage holding means and the voltage generating means. An analog/digital conversion device according to claim 1, comprising circuit means. (4) The voltage generating means adjusts the time for applying voltage to the time constant circuit means to adjust the reference voltage held by each voltage holding means.
) The analog/digital converter described in item 2.