Zero and Gain in a HED Signal Amplifier Board

Short pins 3 and 4 together and adjust "zero" for 0 mv output. Then apply a signal source (e.g. square-wave at 1 mv amplitude) and adjust "gain" so that the output is 100 mv amplitude. Then short the input (pins 3,4) together again and adjust "zero" so that the output is zero.

There are two basic configurations of operational amplifier connection...

1/ Non-inverting where input goes to + input of amp and feedback is to - input from output via a resistive divider, cold end of which goes nominally to input common, but can be connected to a zero offset potentiometer or voltage. It is easy to have a high input impedance, since resistor to common can be very high if input bias current is low, for low offset voltage due to op-amp bias current flow through input load resistor.

2/ Inverting, where + input is connected to common along with signal common, while input goes via resistor to input (-) along with negative feedback resistor from output. Input impedance is limited by chosen input resistor.

In case 1/, the position of the offset pot gives variable effect on feedback resistance ratio and hence gain. Zero has to be adjusted with input set to zero, then gain has to be checked by inputting full-scale input volts & adjusted. Zero is then input again & its pot adjusted. Then gain again, back & forth until zero & gain are both as required within negligible error.

In case 2/, the op-amp (-) input is "virtual earth" [same potential as (+) input of op-amp] & zero & gain settings are almost independent of each other if the op-amp input offset is small.

Most likely, you will find "zero" & "gain" are not independent. In your case, you would be inputting zero volts and 2 x 50 mV = 100 mVfor full scale and adjusting for outputs of 0V & 10V.

You might find this HED does not give 0V out when primary current is zero and adjust zero pot to compensate - which upsets gain a little. For best accuracy, you than have use text box V input adjusted to give zero output then check 100mV test box increase raises output to 10V, not 9.9 or 10.1 say.

The repeated zero then gain adjusts can be tedious, but work in the end - there is a limit to fineness of zero/gain pots which may have "play" & jump from one setting to another if they are multi-turn pots or be too coarse if single turn.

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