# Analog signal conditioning

#### fluxanode

Joined Mar 23, 2014
24
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?

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#### MrChips

Joined Oct 2, 2009
30,814
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.

#### Ian0

Joined Aug 7, 2020
9,822
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?

#### MrChips

Joined Oct 2, 2009
30,814
There is a simple option.

#### fluxanode

Joined Mar 23, 2014
24
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.

#### fluxanode

Joined Mar 23, 2014
24
There is a simple option.
need the full resolution of the op amp.

#### Ian0

Joined Aug 7, 2020
9,822
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.

#### fluxanode

Joined Mar 23, 2014
24
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?

#### Ian0

Joined Aug 7, 2020
9,822
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.