Discussion of the voltage reference for the POT2x and POT4/POT4a boards.
Potentiometer or Strain Gangue Amplifier
The POT2b were originally designed to provide current gain, filtering, (and possibly small voltage gain) to potentiometer. Generally for a POT it's signal is typically close to 100% of the applied voltage and so the voltage gain is set to 1x. Because of the relatively high impedance of a POT (10K), a relatively large voltage reference (~ 10V) is applied across the POT create a large voltage signal.
Strain gauges typically produce small signals (1% of the applied voltage) and also have relatively small impedances (350 ohms for a typical bridge). The to product a large output single, the gain of the POT2 needs to be increase to at least 100x.
The reference chosen for the POT2 series is a shunt type, which means that it behaves like an ideal zener diode. Unlike "normal" references, the shunt reference consumes a fixed supply current - independent of the load attached. The shunt regulator design is basically a load specific design - and a strain gauge and POT present very different loads.
The short venison of this story is that the only way to make a POT2 board work with a 350 ohm bridge, it to reduce the voltage reference to something close to 2.5V - thus requiring a further increase in the gain.
Vref = 2.5V + 2.5*RR2/RR3 RR2 = 7.32K, RR3 = 2.49K -> Vref = 9.85V
By switching RR2 and RR3 (RR2 = 2.49k, and RR3 = 7.32k) a lower Vref of 3.35V is achieved.
Also, the maximum supply current available to the reference load is controlled by RR1 and RR4.
More to follow.
Instrumentation amplifier gain resistor is RA1. When working with potentiometers this resistor is left open (unpopulated) and results in a gain of 1x. (An additional gain of 2x is the result of a differential output from the DRV135).
The gain equation is dependant on the instrumentation amplifier (UA1):
- INA128 : G = 1 + 50K / RA1
- INA122 : G = 5 + 200K / RA1
- LT1167 : G = 1 + 49.4K / RA1
The gain resistor RA1 is a 1206 package.
More to follow.