Universal Signal Conditioning Circuit

Here is a practical application of the dual differential amp. This circuit is for DC or slow moving DC like Strain Gauge and transducer signals. The RC and Zeners are for protection and clamping. Why and how has been described in other posts here. The high impedance op-amp input cannot face the real world unprotected.

Opamp Instrumentation Circuits

OP07 is always the selection due to the low offset. When the sensor generates data in mV, you cannot afford any error introduced in this front end, right. Here OP290 is used for better performance. This dual opamp is better for building a two wire 4-20mA transmitter. But Op07 is fine for a three wire transmitter which consumes more power.

Observe the Power supply decoupling caps, this is a rule and every chip needs a power cushion. The front end zeners and caps have to be low leakage in many applications, ensure you have such parts or test them.

There are two trimpots, bourns multiturn PCB trimpots. One for Offsett-Null or Zero and other for Span or Gain. You have to tweak them in a alternating sequence as they are interdependent to some extent.

Now your weak transducer with a voice in mV and uV; can meet an A/D “Eye to Eye” and let the whole world know what is the Weight, Temperature, Humidity, Pressure or Moisture Content in the Harsh Real World full of uncertainties.

This is an ideal circuit for sensor amplification. Strain Bridge circuits too. It is an instrumentation amplifier, means precision. High input impedance to a differential input signal source. The gain can be set with external resistors to over 1000. CMRR trim to match resistor network.

I measure Instrumentation Amplifier like this – Tera Ohms Impedance, Pico Amps Bias Current. Micro Volts Offset Voltage. Here is a Interactive demo i built for such a configuration. The Input Impedance of this module is very high and is symmetric. This circuit can be used for strain gauges and for four wire measurements. If inputs are in mV use OP07. The merit is that it uses only 2 OpAmps yet has high differential Input Impedance.

The Outputs of Opamps are low impedance but still have limits they cannot drive more than a few mA of Current into the Load. If low ohmic value loads are to be applied use external transistors as amplifiers. If inputs Vn-Vp are floating, outputs may be random or oscillating, it is good to have a bias network of 10M resistors to a potential even zero or common, this enables some Vout when input floats.