Hello, I am trying to use a ADC with +- 6volt inputs and need to measure 0-5 volt signals. Is there a way to use an opamp to scale 0-5v input to -6 and +6 volts output for the ADC input?

 

Memorize this basic differential op-amp circuit.



In general, R1 = R2 and Rf = Rg.

Voltage gain Av = Rf / R1 = Rg / R2

This can be written as
Vo = Av x (V2 - V1)
of
Vo = -Av x (V1 - V2)


We can add to our knowledge base this summing amplifier circuit:



With a combination of these two op-amp configurations, we can add or subtract different inputs by summing in the inverting and non-inverting inputs and at the same time apply some voltage gain.

Thus to scale 0-5V to -6 to +6V, you need a gain of 2.4
Then you need at add/subtract a DC bias in order to shift the output to the desired DC offset.

Hope this helps you.

 

What is the part number of the ADC? Have you checked whether there might be a way of setting it to accept 0-5V inputs?

 

There is a simple option.
Just connect the 0-5V input to your ADC. You will be using about 40% of the ADC range.

 

What is the part number of the ADC? Have you checked whether there might be a way of setting it to accept 0-5V inputs?

It is the ADS1118. Actually it is adjustable but all gain options are +-. The voltage is actually +-6.144v . I typically use 4-20 ma to 0-5v conversion with shunt resistor and an opamp I just couldn't figure how to get a neg offset and adjust the gain.

 

There is a simple option.
Just connect the 0-5V input to your ADC. You will be using about 40% of the ADC range.

need the full resolution of the op amp.

 

It is the ADS1118. Actually it is adjustable but all gain options are +-. The voltage is actually +-6.144v . I typically use 4-20 ma to 0-5v conversion with shunt resistor and an opamp I just couldn't figure how to get a neg offset and adjust the gain.

If you look in Section 7.1 of the datasheet, you will see that the lowest permissable input voltage without damage is -0.3V, so connecting it to -6V will break it.

 

If you look in Section 7.1 of the datasheet, you will see that the lowest permissable input voltage without damage is -0.3V, so connecting it to -6V will break it.

Look at table 3 Full-Scale Range and Corresponding LSB Size?

 

The ADS1118 measures differential signals, so it can measure ±6.144V difference between two pins, but neither of those pins may go below -0.3V or above +5.5V, so it can really only manage ±5.8V.
If you switch to the 4.096V range, it can measure ±4.096V between the pins at full resolution. If you use a single ended input, you can only get 15bit resolution. You have to decide if using an op-amp to generate the other half of a differential signal will give you the extra 1-bit of resolution, or whether the noise of the op-amp will lose it. Remember that 1 bit resolution on a 16-bit converter is a signal to noise ratio of 96dB. (1 bit in 15 bits is 90dB) With an op-amp it might not be that good.

Look at table 3 Full-Scale Range and Corresponding LSB Size?

I have - but Absolute Maximum Ratings trumps everything.

 

ok thanks for the clarification