Hi,
I am getting confused with my first circuit with an op-amp. I am trying to voltage-amplify a signal from a PT100 sensor using an LM358AM dual op-amp, see attached schematic.
For some reason, the 117mV input on the voltage follower causes a 180mV output. As a result the inverting amplifier correctly sets 4V at its output.
Increasing the input voltage above 180mV, the voltage follower starts working as I expect.
Reaching 235mV, the inverting amplifier fails to keep the balance as it cannot decrease the output further. At 245mV input voltage I measure 12,7mV at the output, so the inverting amplifier is able to go under 20mV with its output.
So what is the reason that the voltage follower can not? At this point it is sinking less than 1mA current, which it should do without problem.
(Obviously I tried to replace the op-amp, but obviously it did not help, so the op-amp is OK.)
Hope somebody can point me to what I do wrong. Thanks.
Kralg
I am getting confused with my first circuit with an op-amp. I am trying to voltage-amplify a signal from a PT100 sensor using an LM358AM dual op-amp, see attached schematic.
For some reason, the 117mV input on the voltage follower causes a 180mV output. As a result the inverting amplifier correctly sets 4V at its output.
Increasing the input voltage above 180mV, the voltage follower starts working as I expect.
Reaching 235mV, the inverting amplifier fails to keep the balance as it cannot decrease the output further. At 245mV input voltage I measure 12,7mV at the output, so the inverting amplifier is able to go under 20mV with its output.
So what is the reason that the voltage follower can not? At this point it is sinking less than 1mA current, which it should do without problem.
(Obviously I tried to replace the op-amp, but obviously it did not help, so the op-amp is OK.)
Hope somebody can point me to what I do wrong. Thanks.
Kralg
Attachments
-
19.3 KB Views: 33