Instrumentation amplifier

[NARENDRA1234]

how to see what is input resistance of IA in the data sheet?whether its internally present or not?see the datasheet....

 

[keith1200rs]

That isn't an instrumentation amplifier. Also, don't upload datasheets - link to them on the internet.

On page 2:

Common Mode Input Impedance ZCM — 10^13||6 — Ω||pF
Differential Input Impedance ZDIFF — 10^13||3 — Ω||pF

Keith.

 

[NARENDRA1234]

ok..but we can use it instrumentation amplifier..if somebody asks about its input impedence then what is answer?

 

[keith1200rs]

The way you connect an opamp can change its input impedance, but otherwise it will be as I said above. It is in the datasheet.

Keith.

 

[NARENDRA1234]

we have connected in instrumentation mode...its resistance not given in the datasheet..

 

[keith1200rs]

As I said earlier:

On page 2:

Common Mode Input Impedance ZCM — 10^13||6 — Ω||pF
Differential Input Impedance ZDIFF — 10^13||3 — Ω||pF

Keith.

 

[NARENDRA1234]

what is the meaning of Ω||pF?tell me

 

[keith1200rs]

10^13 ohms in parallel with 6pF.

Keith.

 

[NARENDRA1234]

why parallel C is connected to differential input?

 

[keith1200rs]

It is describing what the common mode impedance of the input pins look like - a resistance in parallel with a capacitance. It is not a case of there being a capacitance "connected" to the input - the input has a capacitance due to the devices in the chip and the pins.

Keith.

 

[NARENDRA1234]

thank you very much..what if we connected capacitor to input side?i am asking these questions because we hav:razz:e final oral tomorrow....

 

[LvW]

we have connected in instrumentation mode...its resistance not given in the datasheet..

Why don't you show us the circuit diagram? How could somebody answer your question without any information about the circuitry?

 

[FvM]

No matter, if you implemented figure 4-10 or figure 4-11 instrumentation amplifier scheme from the MCP604 data sheet, the input resistance of each input will be almost exactly the ZCM common mode input impedance given in the DC characteristics. As a result of the IAs internal feedback, the impedance of each terminal isn't effectively affected by the other input.

In so far, the input impedance of an instrumentation amplifier can be clearly derived from the datasheet, requiring some general OP circuit knowlegde however. You can refer to a text book, e.g. Gray/Hurst/Lewis/Meyer, that has a detailed discussion of differential and common mode input impedance. But it's possibly too late to start reading it now.