Hello dear support team,
Could you please help me in answering the following questions:
BR
Hello BTS,
Thanks for reaching out on the forum. The Ref Resistors are real and as indicated in the figure below are roughly 50kΩ. Their tolerance is 15%, however they are trimmed to match.
The common mode impedance is 500kΩ while the differential impedance is about 3.2kΩ. For calculating the common mode impedance, you take two point on the upper linear portion of the input bias current vs common-mode voltage plot. In this case I chose (20V, ~100μA) and (70V, 200μA). Thereupon you get the difference between these two points (50V, 100uA). Lastly, you divide your derived voltage (50V) from the derived current (100μA) to get 500kΩ.
As for the differential impedance, you first take the difference in current between a point on the IB+ line and a point on the IB- line of the Input Bias Current Vs. Differential Input Voltage Plot. In this case I take values at a 1V differential voltage. So I get 240μA – (-70μA)=310μA. Thereupon I divide the differential (1V) by this difference (310μA), to get 3.2kΩ.
The PWM rejection is more complicated than just an EMI filter. The design engineers took special steps on each stage of the INA240 circuit, to attain that special PWM filtering. As it involves the whole circuit, there is too much proprietary detail involved to share on the forum.
As for using the ref pins as a source, what exactly were you planning on doing?
