20KHz Instrumentation Amplifier

Hi,

please make a scope picture of both input and output voltage.

Klaus

If the output of the opamp with a gain of 6 has a triangle waveform at 350kHz then its input (the coils or the wiring to the coils) is picking up 350kHz from something nearby.

FvM said:

Instrumentation amplifier inputs must not float.

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Hi,

Thanks for your valuable feedback.
Would you please tell me how the inputs would be connected?

Thanks & Regards.

Would you please tell me how the inputs would be connected?

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You can bias the coil center tap as in post #1 (in this case to ground), or use bias resistors. In any case the input voltage must be within the common mode range.

FvM said:

You can bias the coil center tap as in post #1 (in this case to ground), or use bias resistors. In any case the input voltage must be within the common mode range.

Click to expand...

Hi,

Thanks for your valuable feedback.
I have two RX coils, each of which has inductance of 1.7mH and which are connected series opposite. I have connected them with an instrumentation amplifier.
There is a Tx coil ( which is not shown here), which is transmitting the EM field of 16KHz continuously. The two RX coils should cancel out this signal and give a ZERO output at the output of the instrumentation amplifier output terminal. But, should give a NON-ZERO value if there is a metal in-between the TX and RX coils.
I have designed it in Hardware, but I am getting ZERO output from the at the output of the instrumentation amplifier both the cases ( NON metal and presence of metal).
I am using +12V and -12V supply.
I would be highly obliged if you tell me the following points.

1. What should be the value of Vref in this regard?
2. What would be the value of bias resistor?

Thanks & Regards.

The datasheet for the LT1167 and most other opamps and instrumentation amplifiers show the input common-mode voltage range which are the voltages that the inputs work properly. For the LT1167 it is the negative supply plus 1.9V to the positive supply minus 1.4V. Your inputs have no DC bias voltage so of course they do not work and might even be destroyed.

To fix it connect the junction of the two coils to 0V. then you do not need a bias resistor.
If Vref is 0V then the output will swing positive and negative around 0V. I cannot remember what the circuit is driving.

For single supply Vcm must be <2V away from supply rails ( 1.2~1.6)
For split supply same applies, but 0V is natural Vref for Vref centre tap.


No bias R needed,

Lots of gain needed though.