I have a LT1001 op amp wired as a voltage follower. +V is +5 and -V is GND. The inverting input is connected to the output and i have a pot between pins 8 and 1 to adjust the offset (pot is 20k with wiper to +5).

When I bring the non-inverting input to GND the lowest output voltage is about 500mV. I don't believe its a rail-to-rail type of opamp, but does this sound correct or is my chip bad?

If its supposed to be like this, how can i trim that out? The 20k offset pot doesnt seem to make much of a change, the only changes I see are when the input rises above that 500mV threshold.

 

hi MF,
As you know the OPA is designed for dual supply operation.
Checking the d/s shows that 500mV is the closest you will get with a single supply.
To get to 0V out you require dual supply voltages.
One crude method I have seen used is a diode in series with the output, this will drop ~0.7v, into a 4k7 load.
E

 

Hello,

The opamp is not rail-to-rail.
If you want to have the output go to 0 Volts, make the -V connection negative.

Bertus

 

It's not that the chip is bad, it's that you're using the wrong chip. The LT1001 was NOT designed to operate off a single +5V supply, and with a supply voltage that low it will barely twitch, if it does anything at all. As a voltage follower, it will be completely useless.

Before you use any op amp in a circuit, READ ITS DATA SHEET to make sure it will function the way you want. You absolutely CANNOT ignore such specs as Input Common Mode Range and Output Voltage Range in any design, especially a voltage follower operating on a single, 5V supply. For single-supply operation, those two specs are critical.

You need to use a RRIO (Rail-to-Rail Input and Output) op amp. One such part is an LMC6482, although there are many others.

 

Interesting...Ok. I have dealt with digital electronics extensively but just started getting into the analog side of things. I am currently playing with precision instrumentation type circuits and learning more about these. Thanks!

 

@OBW0549 I took your advice and searched some parts bins at home. I got much better results using the LTC6081 which is a Rail to Rail In/Out type. Lowest Voltage I can reach is 0.001250. Data sheet says a 70uV offset but...I'm utilizing vectorboards for testing so I cant expect too much.

Thank you guys!

 

Excellent! Sounds like you've fixed the problem.