Electronic – Instrumentation amplifier: function of resistors at the input
instrumentation-amplifierresistorsseries
I was wondering:
what is the function of the 2 resistors(R4 and R5) in series with the input pins of the given Instrumentation amplifier?
Thank you!
Best Answer
Short answer : Safety current limit for CM input range input Vcb forward bias current from 0.22Ω* current pulse as the source during OFF commutation or unintended loose wire contact when running.
Also ground shift from poor wiring ESL. where Vemf=Ldi/dt for sense wiring L
Normally the shunt R 0.22Ω is designed for 50mV drop at max rated current to keep I^2R losses low. But as we know Motor DCR cause start currents >8x the Imax rated current, the negative voltage on the current shunt may exceed -100mV rating for CM input range on this INA.
If it exceeds -600mV then the input Vbe of the PNP base collector begins to conduct and will blow the front end. So a wise choice of R value is to limit the current with some experience.
Negative voltages occur from back EMF when the motor is shunted to stop, then it acts as a generator and V = L dI/dt depends on both conduction current and rate of change of current (negative) switching when motor is turned Off also resulting in a negative current spike.
You didn't show the all-important value of C1, but it together with R1 looks more like a low pass rolloff than a deliberate phase shift. Without knowing C1, we can't tell what the rolloff frequency is, and therefore whether it produces significant phase shift over the valid frequency range or is just there to cut down high frequency drive to the right leg.
The basic principle is to drive the right leg so as to null out the common mode variations in the signal accross the heart. The body is going to pick up whatever the ambient electric fields are, particular the power line "hum".
The signal to noise ratio of skin voltages is horrendously bad. The common mode power line hum can easily be a few orders of magnitude higher than the signal you are trying to measure. A ideal inamp amp will eliminate that, but nobody has made one of those yet. They all have some real upper limit on common mode rejection and common mode range. By trying to null out the common mode part of the signal, it helps the inamp do its job better. If nothing else, it cuts down on the common mode range that the inamp must be able to handle, even if common mode rejection ratio isn't the main issue.
Yes you can do that. There's not much point in using an instrumentation amplifier when the input is single-ended but there's no harm in it (other than to your pocketbook and likely inferior noise performance). Since you're preceding it with a gain of 1000, noise is not going to be an issue, so just cost.
I do hope your circuit is AC coupled, otherwise you're going to have problems- the 250uV offset of the INA126 multiplied by 10^6 is 250V which will rail the output amplifier.
Best Answer
Short answer : Safety current limit for CM input range input Vcb forward bias current from 0.22Ω* current pulse as the source during OFF commutation or unintended loose wire contact when running.
Also ground shift from poor wiring ESL. where Vemf=Ldi/dt for sense wiring L
Normally the shunt R 0.22Ω is designed for 50mV drop at max rated current to keep I^2R losses low. But as we know Motor DCR cause start currents >8x the Imax rated current, the negative voltage on the current shunt may exceed -100mV rating for CM input range on this INA.
If it exceeds -600mV then the input Vbe of the PNP base collector begins to conduct and will blow the front end. So a wise choice of R value is to limit the current with some experience.