K8055 Analog Input Questions

Hello there!

I hope some of you can help me out a bit here and point me in the right direction.
I am trying to measure a strain-gauge with the K8055 at the analog input.

The circuit is quite simple. I use a 5V source-voltage (Voltage-Regulated) which is connected to a Wheatstone-Bridge (built with 500k OHM resistors). The strain gauge is added to one arm of the bridge so the small resistance changes of it lead to a slight imbalance of the whole bridge. The output of the bridge is then used as input for an INA122PA operation amplifier.

Well to make things quick. When I measure the output of the INA122 directly with a voltmeter I get good stable readings from 0 to 5V when straining the gauge etc. However when I connect this output to the K8055 I don’t get any readings at all. And yes, I have removed the jumpers on the K8055 and I have turned the Potentionmeter to check if it was set wrong.

In fact, connecting the voltmeter and then connecting the K8055 instantly changes the measurement of the voltmeter and I can see that the moment I connect the K8055 the voltmeter readings go down to like around +/-0.5V and no matter what I do the voltage does not change. Not when straining the gauge, not when changing the amplification, nothing…

To test if the analog-input of the K8055 works, I built a simply voltage-divider circuit and connect it directly to the K8055. There I do get good readings. It is just when I use the INA122 output (millivolts and microvolts amplified to volts) that I don’t get any readings at all…

My resistors for the Wheatstone Bridge are quite big, around 500k. At that time I thought it quite a good idea to use those resistors to reduce the current of the circuit.

My question now for all you experts is …

  • Can it be that since I built a bridge with 500k resistors, that the input (and output) current from the INA122 is too low to make any measuring by the K8055 possible?
  • Might this be the reason why a simple lower resistance 5V input voltage divider leads to workable results?
  • If that might be the reason, what is the minimum current that is required by the K8055 for good measurements? What would you aim for?

Or if I am completely thinking in the wrong direction and you have more ideas whats going wrong?

Any help is very very much appreciated! If you need more information, just ask and i will try to explain in more detail!

The input resistance (load) of the K8055 analog input is 101k. This is the result of the trimmer potentiometer (RV1, RV2) and a 1k series resistor (R3, R4).
There should not be any problem for the operational amplifier INA122 to feed this 101k load.
You may test this by a separate 100k resistor. Connect the resistor similar way to the INA122 as you connected the input of the K8055 and measure the voltage over the resistor.

  • Maybe you connected the K8055 input between amplifier’s REF pin and the output pin? You should connect the negative supply of the INA122 to the GND terminal of the K8055.

As an example please see “FIGURE 6. Single-Supply Current Shunt Measurement” in the datasheet: ti.com/lit/gpn/ina122