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.

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.

Opamp Instrumentation Circuits

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.

Linearizing Circuit with Opamps

The thermocouples and some other sensors have an output that is non-linear. That means the mV generated by them are not in direct proportion to the parameter measured. In signal conditioning Linearization can be done with cmos fet switches or even comparators and diodes.

In Embedded Systems Linearization is done with Look-Up tables on ROM and also using Math and Formulae for each segment.

The above circuit used diodes and opamps to change the gain for each segment of the curve to make it a straight line within an acuuracy of 1% or 2 %.

LM346 has programmable gain and 4 opamps in one package. The LF442 is a dual low power operational amplifier and superior for DC application in comparison  to LM358.

The about configuration increases or decreases the gain of the block. You can use each opamp and the dip-switch to change the gain. The resistor selection is important for obtaining the right gain.

Auto ranging 4-1/2 Digit Digital Voltmeter

Auto Ranging for ICL7107 had to be done with window comparators. In ICL7135 Digital Outputs are available for autoranging. Here in this circuit is a close approximation of what i may have designed once.

Auto ranging 4-1/2 Digit Digital Voltmeter

The below Details from Datasheet - These pins are used in this circuit to Autorange the DPM with some peripheral CMOS Logic.

"OVERRANGE (Pin 27) - This pin goes positive when the input signal exceeds the range (20,000) of the converter. The output F/F is set at the end of BUSY and is reset to zero at the beginning of reference integrate in the next measurement cycle.

UNDERRANGE (Pin 28) - This pin goes positive when the reading is 9% of range or less. The output F/F is set at the end of BUSY (if the new reading is 1800 or less) and is reset at the beginning of signal integrate of the next reading."

ICL7135 4 and Half A to D converter with BCD Output

The Range is Defined by QB and QC of 4029, there are four ranges. The two bit Code from this can control the decadic range of an Attenuator or Amplifier.

The Overrange and Underrange are in a OR formation with R26-27. Clock from either will Inc/Dec 4029. The Underrange Pulses are used by D2 -C8 to form a Steady Logic signal on Up/Down Control of 4029. This works in a closed loop manner, to Change the Range to match input Signal. When there are no Pulses from Either UR or OR, then that range is held.