JPH0528833Y2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Field of Industrial Application The present invention relates to an A/D conversion circuit, particularly an A/D conversion circuit called a non-feedback comparison type.

(b) Prior Art The A/D converter circuit of the type mentioned above has been used in various fields because of its simple configuration.
Generally, the configuration is as shown in FIG. That is,
The figure shows the case of converting an analog signal into a parallel 2-bit, ie, 4-gradation, digital signal.
~3 is a voltage comparator, R1 to R4 are voltage dividing resistors that determine the threshold level of each comparator, and 4
is a binary encoder.

By the way, the A/D conversion circuit shown in Figure 1 is used in image processing devices such as personal computers to digitally process video signals from various video devices such as television receivers and VTRs. However, the DC level (brightness) of the video signal
and amplitude (contrast) differ depending on the video source, the voltage dividing resistors R1 to R
If each reference voltage according to 4 is fixed,
It is impossible to always use the full dynamic range of the A/D conversion circuit for each input video signal.

For this reason, for example, the second issue of Japanese Patent Publication No. 57-5094
As shown in the figure, R1 or R of the voltage dividing resistor
It is conceivable to replace 4 with a variable resistor to adjust each reference voltage. For example, in Figure 3, resistor R4
is a variable resistor, and represents the change in each of the reference voltages V1 to V3 when the current i flowing through each of the resistors R1 to R4 is changed by adjusting the variable resistor. Therefore, as can be seen from the figure, even with this method, the dynamic range is used to the full for a video signal with high brightness and low contrast as shown in Figure 2. D conversion is not possible. This is because, in this case, each of the voltages V1 to V3 cannot be set sufficiently close to the high voltage side.

(c) Purpose of the invention This invention was created in view of the above points,
It is an object of the present invention to provide an A/D conversion circuit that can use substantially the full dynamic range for each of various analog signals having different DC levels and amplitudes, thereby enabling efficient digital conversion. .

(D) Structure of the invention The A/D conversion circuit of the invention includes a first voltage generating circuit that increases (decreases) for a while after the start of operation and then is held at a constant voltage, and a circuit that generates a first voltage that is higher than this first voltage. A circuit for creating a second voltage that is always kept low (high) at a constant voltage for a while after the start of the operation, and then increases (decreases), and a voltage between the first and second voltages at a constant ratio of 2 ⁿ − A voltage dividing circuit that divides into three voltages is provided, and each output voltage of the first and second voltage generating circuit and the voltage dividing circuit is connected to each reference voltage of 2 ⁿ -1 voltage comparators provided on the input side of the encoder. This configuration is characterized in that the voltage is applied as follows.

(e) Embodiment FIG. 4 shows an embodiment in which an input analog signal is converted into a 3-bit, 8-gradation digital signal. In this example, VR is resistor R1, R2
a variable resistor, which cooperates with the variable resistor to create a variable DC voltage;
Tr1 is a first transistor for isolation so that the voltage obtained at the slider is not affected by the subsequent circuit, Z1 is a Zener diode that constitutes the first voltage generation circuit 11 together with resistor R4, Z2, Z
8 is a second voltage generating circuit 12 along with resistors R5 and R6.
Tr2 and Tr3 are the same second and third transistors (for isolation) as described above for each voltage obtained at points b and c, R9 to R14,
R9=R10=...R14 is a voltage dividing resistor for creating a reference voltage, 13a to 13g are voltage comparators, and 14 is a binary decoder.

In the embodiment of FIG. 4, now the variable resistor
The VR slider is installed from the ground side to the +B power supply side and is operated by sliding. Then, if the voltage obtained at the slider is Vr, the voltage at point a is Va=Vr
−Vbe, and this voltage Va is given by diode Z
While the diode is off at a voltage lower than the Zener voltage Vz1 of 1, the voltage Vb at point b is equal to the Va which changes as the variable resistor VR is operated. However, when Va becomes higher than Vz1 and the diode Z1 turns on, Vb becomes Vb=Vz1.
and keep it at a constant value. Therefore, Vb is the fifth
It will change as shown by straight line A in the figure.

On the other hand, diode Z3 is connected to +B by resistor R5.
It is biased from the power supply and is always on.
Therefore, the voltage Va is calculated as follows, assuming that the Zener voltages of the diodes Z2 and Z3 are Vz2 and Vz3, respectively.
When ≦Vz2+Vz3, the diode Z2 is turned off, so the voltage Vc at point c is Vc=Vz
It is held at a constant value at 3. However, Va>Vz
2+Vz3 and the diode Z2 turns on, Vc becomes Vc=Va-Vz2=Vr-Vbe-Vz2
As a result, it changes as shown by straight line B in Figure 5.

Here, the voltages Vo and Vp at point O and point P are respectively
Since Vo=Vb+Vbe・Vp=Vc+Vbe, the potential difference between the O and P points (Vb-Vc) and the median potential 1/2 (Vz1+Vz3) (see Figure 5)
is set in consideration of the amplitude of various analog signals input to the input terminal 15, the range of change in the DC level, etc. Then, the voltages Vo, Vp at the above O and P points
The voltages obtained by dividing the voltage between the two by resistors R9 to R14 are applied as reference voltages to the voltage comparators 13a to 13g.

Therefore, the voltages Vb and Vc are set to voltages within the range shown in FIG. 5 for analog signals with a low DC level and small amplitude, and when the amplitude is relatively large and the DC level is approximately 1/2 of the amplitude. It is sufficient to set the voltage within the range for an analog signal with a high DC level and a small amplitude, and to set the voltage within the range for an analog signal with a high DC level and small amplitude.
Of course, in each case, Vb needs to be set slightly lower than the voltage change range of the analog signal, and Vc needs to be set slightly higher than the minimum value of the voltage change range.

By doing this, it is possible to obtain an 8-gradation digital signal at the output terminals 15a to 15c, which is converted using the dynamic range at almost full capacity.

That is, the present invention is a variable resistor for generating a variable DC voltage, as shown in the claims of the utility model registration.
a first voltage generating circuit connected to the variable resistor VR and generating a first voltage (the voltage shown in FIG. 5A) which rises when the variable DC voltage is in the first and second ranges (the ranges in FIG. 5A) and is kept constant when the variable DC voltage is in the third range (the range in FIG. 5A); a second voltage generating circuit connected to the variable resistor VR and generating a second voltage (the voltage shown in FIG. 5B) which is always lower than the first voltage, is kept constant when the variable DC voltage is in the first range, and rises when the variable DC voltage is in the second and third ranges; and a voltage dividing circuit.

Alternatively, a variable resistor to create a variable DC voltage
VR, and the variable DC voltage is connected to the variable resistor VR, and decreases when the variable DC voltage is in the first and second ranges (areas in FIG. 5), and decreases when it is in the third range (areas in FIG. 5). A first voltage generation circuit that generates a first voltage (voltage shown in FIG. 1, it is held constant when it is in the range of 1, and the second
and a second voltage generation circuit that generates a second voltage (the voltage shown in A in the figure) that decreases when the voltage is within the third range, and a voltage dividing circuit.

(f) Effects of the invention According to the A/D conversion circuit of the invention, the dynamic range of the A/D conversion circuit can be used almost to the fullest for each of various analog signals with different DC levels and amplitudes. can be efficiently converted to digital. Therefore, it is suitable for digitally processing video signals from various video sources.

Further, since the first and second voltages exhibiting different change characteristics can be created by adjusting one variable resistor, the reference voltage can be created with an extremely simple configuration.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a conventional A/D conversion circuit, FIG. 2 is a signal waveform diagram showing its input analog signal, and FIG. 3 is a characteristic diagram for explaining its operation. FIG. 4 is a block diagram showing one embodiment of the present invention, and FIG. 5 is a characteristic diagram for explaining its operation. 11...First voltage generation circuit, 12...Second voltage generation circuit, R9-R14...Voltage dividing resistor, 13a-
13g...Voltage comparator, 14...Binary decoder.

Claims (1)

[Claims for Utility Model Registration] 2 ⁿ to which different reference voltages are respectively applied
- In a circuit that inputs an analog signal to one voltage comparator (n is a natural number of 2 or more) and inputs the output of each comparator to an encoder to convert it into an n-bit digital signal, the variable DC voltage a variable resistor connected to the variable resistor, which increases (decreases) when the variable DC voltage is in the first and second ranges and held constant when it is in the third range; a first voltage generating circuit that generates one voltage; a second voltage generation circuit that generates a second voltage that increases (decreases) when in the second and third ranges; and a voltage between the first and second voltages at a constant ratio of 2 ⁿ −
A voltage dividing circuit that divides the voltage into three voltages is provided, and each output voltage of the first and second voltage generating circuits and the voltage dividing circuit is applied as a reference voltage to each of the comparators. /D conversion circuit.