Hi Folks,
I've got a shunt (75mV/300A) and built a small op-amp circuit to bring the 75 up to 2.8V. Before you connect the shunt, the input is floating and as such, the op-amp has an output.
How do you determine the value of a pull down resistor? When the op-amp's input is floating, the output is 3.6V (curious as to why that voltage) so "Simples" I said, add a pull down resisitor. I first opted for a 1M thinking anything less would skew the output from the shunt.
I decided to try a 220K, put it in and tested. The output when the pin was not connected to the shunt (yet not floating) was 300mV. When connected it seems to work ok.
if I short the input to ground, the output is zero.
Seeing as 300mV is still not zero, how would I calculate what pull down is needed which will a) hold the output at zero and b) not affect what the shunt is trying to do?
TIA
Cheers,
Crispin
I've got a shunt (75mV/300A) and built a small op-amp circuit to bring the 75 up to 2.8V. Before you connect the shunt, the input is floating and as such, the op-amp has an output.
How do you determine the value of a pull down resistor? When the op-amp's input is floating, the output is 3.6V (curious as to why that voltage) so "Simples" I said, add a pull down resisitor. I first opted for a 1M thinking anything less would skew the output from the shunt.
I decided to try a 220K, put it in and tested. The output when the pin was not connected to the shunt (yet not floating) was 300mV. When connected it seems to work ok.
if I short the input to ground, the output is zero.
Seeing as 300mV is still not zero, how would I calculate what pull down is needed which will a) hold the output at zero and b) not affect what the shunt is trying to do?
TIA
Cheers,
Crispin