Hello,
Can you help clarify the closed loop gain from the LMV641 datasheet? In equation 4, it’s listed the required gain is 23.2 while in equation 6 where user calculate the max frequency, we advised to use the gain of 26.3. In the case of this example which should be the correct closed loop gain?
Thank you,
Raphael
The difference in an original gain of G=23.2 and the one used for the actual calculation of MAX FREQ, G=26.3, comes from the need to use an additional gain stage, U2, to compensate for the negative temperature drift of a resistive bridge - see below:
U2 is an additional gain stage with a thermistor element, RTH, in the feedback loop. It performs a temperature compensation function for the bridge so that it will have greater accuracy over a wide range of operational temperatures. With mangetoresistive sensors, temperature drift of the bridge sensitivity is negative and linear, and in the case of the sensor used here, is nominally −3000 PP/M. Thus the gain of U2 needs to increase proportionally with increasing temperature, suggesting a thermistor with a positive temperature coefficient. Selection of the temperature compensation resistor, RTH, depends on the additional gain required, on the thermistor chosen, and is dependent on the thermistor’s %/°C shift in resistance. For best op amp compatibility, the thermistor resistance should be greater than 1000 Ω. RTH should also be much less than RA, the feedback resistor. Because the temperature coefficient of the AMR bridge is largely linear, RTH also needs to behave in a linear fashion with temperature, thus RA is placed in parallel with RTH, which acts to linearize the thermistor.
