Adding offset to output in a cascade topology

Mehran

Hello All,

I am wondering why some people prefer to add the offset to the second stage of the cascade opamps when they want to support bipolar differential inputs. I heard something about input balance in differential analog measurements. in this way, a negative supply is needed to pass the negative inputs from the first stage. (topology 1 )
We usually add offset to the positive input of an opamp to shift the voltage from negative range, hence we can support bipolar differential analog inputs with a single supply opamp. (topology 2)

Could you please let me know if these assumptions are correct and which one is the best regarding the quality of measurements.

Regards,
Mehri

over 5 years ago

0 Ron Michallick over 5 years ago

TI__Guru**** 165926 points

Hello Mehri,

You answered your own question. Common mode input current matching is the reason that I see.

The quality of measurement is based mainly on resistor matching (do not forget to add the source signal impedance). The offset DC voltage of the amplifier also matters. Figure 1 has more resistors and op amps, so it should have more error sources. Figure 2 places an imbalanced DC load on the source, so that might make a bigger error with some input sources, but not others.

0 Mehran over 5 years ago in reply to Ron Michallick

Prodigy 150 points

Thanks a lot,

So this might be one of the reasons that we use fully differential amplifiers?

I measured the FDAs input when they are floating and I saw Vocm on both pins, I would like to know more about this imbalance definition and effect. Do you have an article about it to read?

Since I see also the second topology is widely used so I want to know how do they decide on it.

Regards,
Mehri

0 Ron Michallick over 5 years ago in reply to Mehran

TI__Guru**** 165926 points

Mehri,

This blog may help. https://e2e.ti.com/blogs_/b/analogwire/archive/2013/11/06/what-you-need-to-know-about-cmrr-instrumentation-amplifier-topologies-part-3

Figure 2 shows a standard 3 amplifier setup and figure 3 shows a standard 2 amplifier setup.

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Adding offset to output in a cascade topology