Resistance-to-voltage conversion circuit used in resistance sensors

Function of the circuit

The circuit that converts resistance value into voltage is actually a resistance meter, which can directly indicate the resistance value. The method of measuring resistance is: let a constant current flow through the unknown resistance R, and then measure the voltage across it to find the resistance value of R. There are many circuit forms to implement this method. This circuit does not use a constant current circuit, but connects a resistor of unknown resistance in the feedback circuit of the OP amplifier and uses an equivalent constant current drive. Therefore, this circuit can also be used as a peripheral circuit of a sensor that requires a constant current drive to obtain a voltage output proportional to the resistance value.

How the Circuit Works

OP amplifier A2 is used as an inverting amplifier circuit, and R2 is connected in the feedback loop. If the error sources such as bias voltage V or input bias current IB can be ignored, then the output voltage of A1
is obtained by the formula. The reference resistor R is selected to obtain a fixed reference voltage. At this time, the output voltage EO represents the measured resistance value and is directly read out.

R is divided into 10 times in the full range of 200 ohms to 20 megohms and converted into a voltage output of 0 to 2V. If you want to measure a resistance less than 200 ohms, you should increase the working current. You can replace the current enhancement of the OP amplifier with a transistor that allows a larger collector power consumption. When measuring large resistances of 2M and 20M, R takes 1/10 of the calculated value, and the reference voltage drops to -0.2V.

Reference voltage generation circuit: Use inverting amplifier A1 to attenuate the voltage of about 8.3V to -2.0V. The output current is 10MA at the minimum resistance range. In order to reduce the burden of the OP amplifier, TT1 is used to enhance the current.

Component Selection

The main factors that determine the measurement accuracy are the reference voltage stability, the absolute accuracy of the reference resistor, the output bias current IB of the high-resistance OP amplifier, the bias drift V/△T, etc. Therefore, a low-drift FET type OP amplifier must be selected. Because the stability and accuracy requirements of the reference resistor and the voltage divider resistors R13 and R14 are relatively high, it is best to use a resistor with a resistance error of plus or minus 0.25% and a temperature coefficient of plus or minus 25PPM/degree. The bias current of the Zener diode 1S2192 must be around 10MA to be stable, but it can also be replaced with a 2.5V bandgap reference. At this time, the amplification factor of A1 should be 2/2.5=0.8 times.

Diodes D2 and D3 should be low leakage type, otherwise they will be unstable in the high resistance range. You can also replace 1SS104 with J-FET diodes.

Adjustment

After a few minutes of power-on, adjust VR1 so that -EE=-2.000V, and measure the voltage after voltage division. If it deviates from -2.000V, you can connect a large resistor in parallel to R13 or R14 to adjust the voltage. Offset adjustment of OP amplifier A2: Set the range switch to "6", short-circuit both ends of R14, connect a 10~20M resistor to R, and adjust VR2 so that the output voltage is approximately equal to zero. In addition, you can also set RS=∞ (open circuit) and adjust VR2.