Increase common mode voltage differential adc

[alexscep]

Hello, i am trying to use an differential ADC to measure voltages quite similar to using a multimeter. But to achieve this, i have to extend the common mode input range of the ADC. The CMIR of the ADC used is approximately 10V. But this is not sufficient for my application. So my question is very simple, how can i enlarge this CMIR. It should be possible to measure voltages accross all kinds of components, just like a multimeter.

I thought of using a differential amplifier with high common mode voltage. But the problem with these are that the common mode rejection ratio is highly dependent of the value of the shunt resistor between the two input terminals. Even the slightest input imbalance can cause a large decrease in common mode rejection. So if i want to measure voltage across a high impedance load this would lead to problems.

I have also considered isolated ADC's, but i wanted to know if there is a solution to this problem using non-isolated differential ADC's.

Can anyone help me, or supply me with some reading material concerning this problem?

[dannyf]

The easist would be to use an instrumentation amplifier - it does have its own issues.

Alternatively, use a differential-in / differential-out amplifier - 2 opamps + 3 resistors.

[alexscep]

Would the circuit described in figure 32 on page 17 of this differential input and output differential amplifier (THS414x) do the job?

http://www.ti.com.cn/cn/lit/ds/symlink/ths4140.pdf

And im not realy sure wat you exactly meant with the following:

Alternatively, use a differential-in / differential-out amplifier - 2 opamps + 3 resistors.

Thanks for the fast reply btw

[dannyf]

THS4140 is a very capable opamp. However, for it to work, you will need matching resistors - hard to do.

The circuit I was thinking about utilizes two opamps -> differential inputs are on the non-inverting inputs of the two opamps. The inverting inputs are tied together with one resistor (Rg, to set gains), and each opamp has a feedback resistor (Rf). Rf / Rg sets the gain for the differential output.

The common mode input range would be determined by the opamps' input range (and their rail voltage).

I will see if i can find a picture later. But it is essentially the input stage of a 3-opamp instrumentation amplifier.

[Marco]

But this is not sufficient for my application.

So how much do you need?

[alexscep]

A CMIR of +-40V or wider would be nice.

[Marco]

Just float the thing ... any alternative is going to be much more complex.

[alexscep]

I have to be honest, that i never have tried this before and therefore am completely new to this principle of common mode problems. But what do you actually mean with floating? Could you explain a little further.

[Marco]

Sorry, I mean just use an isolated converter (it floats with the voltages you are measuring, just like a multimeter).

AFAICS without an isolated converter you will always end up using matched resistors to increase the common mode range ... and at that point you might as well just use a difference amplifier and a single ended ADC IMO. They are available for high voltages and the resistors will be much better matched than anything you can do yourself.

[senso]

Maybe this op-amp from LT suites the bill:
Input Common Mode Range: V– to V– + 76V

http://www.linear.com/product/LT6016

[Marco]

Normal opamps are useless here ... he wants to stay fully differential, you can bodge that with resistor dividers but AFAICS there is no real reason to do it. Yes you can say you're staying fully differential through the chain, but practically you're going to be worse off than just using an INA149 and a single ended ADC.

The only way to stay fully differential in a way which makes sense at the common mode voltages he wants is to float the converter IMO.

[alexscep]

Was looking for isolated ADC's, but there aren't that many manufacturers that produce these things. A digikey lookup, gave me a few Texas Instruments ones, but what about this one from Avago Technologies. Seems a decent chip to me.

http://www.avagotech.com/pages/en/optocouplers_plastic/plastic_miniature_isolation_amplifier/acpl-c797/

[Marco]

You don't really need to have one ready made, you can use an isolated power supply for the converter and digital isolators to get the data out. The inputs are connected through bias resistors to 1/2 Vsupply (which will thus be the common mode voltage the ADC sees, as long as the capacitance to ground doesn't play a significant role ... don't use 50/60 Hz transformers for the power supply).

[alexscep]

Ok, so i should better make something shown in the image below. Isolating the analog circuitry from the digital circuitry. Then i use a transformer with two windings on the secondary to achieve an isolated analog and digital power supply?

don't use 50/60 Hz transformers for the power supply

But Marco,you don't approve of using a transformer? What should i use instead then?

It sure isn't easy to find examples for these kind of circuits. Does anyone have a design reference or something similar concerning this subject?

[alexscep]

Or maybe i should use isolated DC/DC converters to provide the supply-voltage.