JPS58209221A - Analog-digital converter - Google Patents

Abstract

PURPOSE:To improve the accuracy and the functional, performance, by detecting an average voltage from an output voltage of a bridge circuit by a control circuit in the proportional comparison system. CONSTITUTION:Four sides constituting the bridge circuit A are provided with resistors R1, R2, R3 and R4 being a distortion detecting element respectively. Output terminals (a), (b) are connected to a control circuit B via a switch E and an amplifier 1. When a load is applied to a load cell, the resistors R1, R2 of the circuit A change according to the load, voltages Va, Vb at the output terminals (a), (b) are changed and outputted to a circuit B for output time t1, t2 controlled with the switch E. The circuit B detects an average voltage Vc, of the Va, Vb. A comparison circuit D outputs each period when the Vc crossing with a waveform of one period of the sawtooth waveform based on the said waveform outputted from an oscillating circuit C. The switch E switches the two output terminals alternately with this output.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an A/D converter that detects the amount of variation in a bridge circuit and converts it into a digital fish.

In conventional A-Di converters, for example weighing devices using load cells, the differential voltage output from the two output terminals of a bridge circuit is directly converted into a digital lid using an integrating circuit using the 2i & minute method. In this conventional method, fluctuations in the power supply applied to the bridge circuit, that is, the reference power supply of the A-D converter, affect the A-D conversion output, so it is necessary to minimize the power fluctuations as much as possible. It won't happen.

Further, the integrating capacitor has drawbacks such as dielectric absorption or leakage current, which deteriorates the linearity of the output, and furthermore, the oscillation frequency of the counting clock oscillator requires an oscillation frequency of N degrees.

In order to eliminate the above drawbacks, it is necessary to use expensive components with a high M degree, such as operational amplifiers, integrating capacitors, reference power supplies, etc., and it is also necessary to adjust the analog circuits, which leads to problems with poor adjustability. will occur.

It is an object of the present invention to eliminate such conventional problems and provide an inexpensive A-D converter with excellent accuracy and functionality. a control circuit connected to two output terminals of the circuit via a switch to detect and output an average voltage from the output voltage of the bridge circuit, and an oscillation circuit that outputs a sawtooth waveform. and a comparison circuit that outputs a time ratio of voltage levels based on the average voltage output from the control circuit and the sawtooth waveform output from the oscillation circuit, and the switch that alternately switches between the two output terminals based on the output of the comparison circuit. Toyoshishi is called % mold.

The average voltage output from the above control circuit does not mean the simple average value of the output voltages output from the two output terminals of the bridge circuit, but the calculated value obtained by multiplying each output voltage and output time by an integrating circuit, etc. means the average value of

To explain an embodiment of the present invention with reference to the drawings, FIG. 1 shows a block diagram of the device, in which (4) is a bridge circuit, (5) is a control circuit, (C) is an oscillation circuit, (2) is a comparison circuit, (6) is a switch, and CF') is a counter.

The bridge circuit (4) is, for example, a load cell of a scale, and a resistor (RzXRxXR2XRz), which is a strain detection element, is provided on each of the four sides constituting the circuit.

Output ends (a), (b)...1 of the bridge circuit (4) are equipped with a switch (g), and the valley output ends (a) Φ) are connected to the control circuit (B). Connecting.

Note that this is the (1) tj amplifier in the figure.

The case where the foot voltage source (2) is applied to the connection points (c) and (d) of the bridge circuit (4) is shown, and the applied voltage (7) causes the output terminals (a) and (b) to have u I = V/R□+ R2
A constant current will flow through the crab.

When a load is applied to the load cell, the resistances (R1) (R2) of the bridge circuit (4) change according to the load, and this causes the output voltages (Va) (Vb) of the output terminals (a) and (b) to change.
) changes, and the voltage (Va ) (Vb
) is output to the control circuit (B).

) of the control circuit is an integral element (
3J, in which element (3), (4) is an integrating capacitor, and (5) is a resistor.

Also, (7) in Figures 2 and 3 is a differential circuit, and (8
) is a proportional circuit, and (6) is an amplifier.

The control circuit (B) is the bridge circuit! M! (4) Output voltage (Va) output from output terminals (a) and (b) (
Vb) and its average voltage (vc) is detected and output.

In other words, the input output voltage (Va XVb ) and output time (tl) (C
2) The voltage (vc)t- at which the calculated values (va-tl, b-t2) become equal is detected and outputted as an average voltage.

The differential element (7) and the proportional circuit (8) in the control circuit (B) have a function of accelerating the convergence of the average voltage (Vc).

The oscillation circuit (C) outputs a sawtooth waveform (c) shown in FIG. 4 and inputs it to the comparison circuit (2), and the period of one waveform is (t) (t=t1+tz).

) of the comparison circuit is connected to the control circuit (in the control circuit) and the oscillation circuit (C), and the average voltage (VC
) and the sawtooth waveform (c) for comparison (Fig. 4), so that at period (1) of waveform (c), waveform (c)
The time (tl) from the initial point (C1) to the level point (C2) where the average voltage (Vc) crosses the waveform, and from the late point (C2) to the final point, that is, the initial point (C2) of the next waveform.
1) is detected and output as a time ratio width signal ψ).

That is, the comparator circuit (2) outputs the average voltage (VC) outputted as a resistance change of the bridge circuit (4) at each time (t1+tz) corresponding to its level.

The switch (6) receives the time ratio width signal ψ of the comparison circuit (b).
), the output terminals (a) and (b) of the bridge circuit (4) are sequentially and alternately switched to the control circuit 0.
3) is connected to the time (tl) (C2) detected in step 3), and the output voltage (Va) (Vb) is output during that time.

When a load is applied to the load cell, the above output operation is repeated, but when the voltage change reaches an equilibrium state, the control circuit ω) calculates the following equation, (R-ΔR) I-h = (R + The formula ΔR)I·C2 is established. Note that R=R1+R2, ΔR is the amount of resistance change.

From the above formula, ΔR=R bow(tltz)/R−I(tl+t
z)=(L+ -tz ＞/t ・・・・・・・・・(
Formula A) In other words, the load applied to the load cell is determined by the comparator circuit C.
D) as a function of the output time (tIXtz).

The time ratio width signal Φ) of the above time (tl Based on the calculated value, it is displayed as a load on the display circuit.

Although the above embodiment has shown the case where the constant voltage source (2) is used as the power supply, the conversion operation is similarly carried out when a constant current source is used as the power source.

The present invention is configured like a rivet, and the control circuit detects the average voltage from the output voltage of the sibridge circuit using a proportional comparison method, so that fluctuation factors in voltage or current cancel each other out, improving output accuracy. Can be done.

In addition, the outputs of the four comparison circuits are fed back to the output end of the bridge circuit to operate the switch, so the dielectric absorption of the capacitor in the control circuit is corrected, the linearity of the output is maintained, and the clock oscillation is a
As long as the degree is stable within one period of the serrated tooth shape, changes over a long period of time due to temperature changes or the like will not affect the A-D conversion accuracy.

Therefore, it is possible to improve the accuracy of the conversion device, eliminate the need for expensive correction components, and provide a highly accurate and inexpensive device.

[Brief explanation of drawings]

FIG. 1 is a block diagram of the device of the present invention, FIG. 2 is a control circuit diagram, box 3 is a control circuit diagram of a modified example, and taJ4 is a waveform diagram showing the operation of the comparison circuit. In the figure, (4) is the bridge circuit, (b)) is the output terminal,
(R1) (R2) are resistors, ω) is a control circuit, (C) is an oscillation circuit, [F]) is a comparison circuit, [F]) is a switch, (
VaXVb) is the output voltage, (VC) is the average voltage, (1)
is the waveform period, and (tx)(t2) is the time ratio of the voltage level. Procedural amendment July 1982? Mortar 1, Indication of the case Patent Application No. 93293 filed in 1988 2, Name of the invention A-D converter 3, Person making the amendment Relationship to the case Patent applicant Name (name) Teraoka Seiko Co., Ltd. 4, Agent Address: 5-14-7 Hakusan, Bunkyo-ku, Tokyo Showa
Year Month Day 6, Full text of the specification subject to amendment and drawing plan 7, Contents of amendment

Claims (1)

[Claims] a bridge circuit having a strain detection element and detecting load as a change in resistance value; a control circuit connected to two output ends of the circuit via a switch to detect and output an average voltage from the output voltage of the bridge circuit; An oscillation circuit that outputs a sawtooth waveform, and each time between the voltage level and the waveform in one cycle of the waveform, based on the Height 8 voltage output from the control circuit and the sawtooth waveform output from the oscillation circuit. An A/D converter comprising a comparison circuit that outputs an output, and the switch that alternately switches the two output terminals according to the output of the comparison circuit.