opamp amplifier for ADC

Hi all,

I have to amplify a voltage signal and read it into a microcontroller. For this I am using the following circuit. R55 and D3 are for clamping the input if it goes beyond 5V.

I had previously connected R38 to the output of the opamp but now decided to connect it between R55 and D3 to compensate for any voltage drop across R55.

My question is will this have any other good or bad effect that I am not seeing?



Regards

hi,
If Vdd is the PIC/MCU supply that should be OK

Usually the PIC's internal diodes will clamp the pin, I sometimes add the extra diode.

E

Isn't an LM358 a bad choice though?, does it go anywhere near it's negative rail?.

R-R

Some of your problems go away if you choose a R-R output amplifier. With the input at or near ground a R-R input part is a good idea. MicroChip has parts. A amplifier running on 5 volts (or what ever the micro is running on like 3.3V) will solve the pull up too much problem. Also it will solve the need for input protection. Remove the 100 ohm resistors.

If you do have a resistor from amplifier to micro and you are concerned about voltage drop: the input current draw on the ADC inputs is +/-1uA. 1uA and 100 ohms is not much error.
------edited------
R-R = rail to rail

If R38 is to compensate the gain for the presence of R55, that's incorrect. Since R55 is inside the feedback loop, R55 will have no noticeable effect on the circuit gain, which will be determined by R36 and R37.

The LM358 requires a load to get it's output near ground. This costs power eg. a 20mA load. @ 5V peak. Comparatively noisy depending on your application. Oversampling by the MCu can mitigate that.

Nigel Goodwin said:

Isn't an LM358 a bad choice though?, does it go anywhere near it's negative rail?.

Click to expand...

The LM358 output goes very close to the negative rail for small output currents, as shown in this excerpt from the datasheet.

crutschow said:

If R38 is to compensate the gain for the presence of R55, that's incorrect. Since R55 is inside the feedback loop, R55 will have no noticeable effect on the circuit gain, which will be determined by R36 and R37.

Click to expand...

As crutshow stated, R55 won't affect the gain. It also won't limit the current as you're expecting (the opamp will increase its output current to its maximum when your clamping diode conducts); you can put it outside the feedback loop, and increase its value.

The op amp is running on 12V. Excessive input voltage can raise the output voltage above Vdd thus raising the regulated power supply above it's intended regulated output voltage due to the way voltage regulators work. The clamping diode therefore does little.

They (the regulators) can source the current but rarely can control the output voltage when their output transistors go reversed biased and therefore can lift the so called regulated output of the regulator.

Something which has been argued here before but taken no notice of by certain moderators despite simulation results.

You'd be better clamping the whole supply with a zener rated above the output of the Vdd reg.