INA199: Internal resistance between Inputs

Hi Archie,

I cannot open your link.

Equation 1 in section 8.4.1 of datasheet of INA199 becomes correct when you assume a 2k5 resistor between the inputs in the "Bias" box in figure 21. "Rint" is shown in Table 1.

The "1250" in the formula is gained by calculation and is just half the 2k5 resistance of the "Bias" box. Also keep in mind that the internal circuit shown in figure 21 is highly simplified. Much more is going on internally than shown.

More, I would be careful with the word "input resistance". "Rint" is the internal resistances at the inputs of INA199 according to figure 21. But this is not the input resistance of INA199. For a pure common mode input voltage, which means an identical input voltage at both inputs without any differential input voltage, each input sees an input resistance of about 1M + Rint. For a pure differential input voltage without any superimposed common mode input voltage, on the other hand, there is a 2k5 resistance between the inputs.

Unfortunately, when you have a mix of a common mode input voltage plus a differential input voltage across the inputs, the situation becomes highly complex and you can no longer talk about a "common mode input resistance" or a "differential input resistance" or even an "input resistance" of INA199. It's better then to talk in terms of input bias currents and input offset current. In the very most cases (with a meaningfully low shunt resistance) the "input resistance" of INA199 is so high that it can be neglected anyway. This does not apply to the input bias currents of INA199, of course, which originate from the "Bias" box in figure 21, by the way.

Kai

Hi Archie,

the "Gain Error Factor" shown in equation (1) of datasheet of INA199 can be found this way:

archie_ina199.docx

Kai