Amplifying the output a pressure transducer can be challenging if you are new to pressure sensors, or if you have little experience to analog circuitry. While there are a number of circuits on the internet that offer a solution, often times the circuit is very complex and does not explain exactly the pitfalls of using that particular circuit. This is even true of the manufactures themselves. A good example of this is the GE/NovaSensor NPC-1210 and the IC Sensors (a division of Meas-Spec) 1210A Series
The Circuit as Shown on the Datasheet
Originally the 1210A was introduced by IC Sensor, and then (a few years later) NovaSensor copied the design and renamed it the NPC-1210 (NPC stands for: NovaSensor Pressure, Ceramic). Unfortunately, when NovaSensor copied the sensor they also copied the datasheet almost verbatim and did not fully describe the circuit.
The circuit shown on both datasheets is shown in Figure 1 below. In this circuit the gain set resistor (Rset) is used to set the gain of the differential amplifier.
Diff Op Amp from NPC-1210 datahseet
Figure I
There is nothing wrong with this circuit, but on all of the datasheets this circuit is shown, the NPC-1210, NPC-1220, 1210A, and the 1230 datasheets, the manufacturer failed to adequately explain how circuit works.
Circuit Gain
The gain of this differential amplified is defined by the following equation:
Gain= Vout/Vin = (1+2*100,000)/Rset
Lost to antiquity is why 3.012 V was chosen as the desired output. However, if you use just use the minimum, typical and maximum sensors outputs, (and assume a zero offset voltage!), the following gains and Rset values will be achieved using the above equation:
The Gain Set resistor (RSet)
Remember that Rset is a stand-alone thick film resistor on the ceramic (therefore a discrete resistor) and is trimmed to a guaranteed interchangeability of 1% (IC Sensors) and 2% (GE) of what is required achieve the 3.012 V amplified output voltage. Each sensor Rset resistor is trimmed using FSO of the actual sensor.
The actual resistor values can range from 1660 ohms to 9960. The 1% and 2% tolerances given by the manufactures is really how close the thick film resistor is trimmed to the theoretical output, given the known FSO of the sensor. It is not the guaranteed value to the Rset resistor from device to device.
Conclusion:
If you need a circuit with a differential output of 3.012V then this circuit is the circuit for you. However, most most application do not require a different output, and at the end of the day, few people use the Rset resistor in this circuit. In addition the circuit makes no mention of how the supply current “I” is actually made, nor how the offset is nullled as is often required.
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