| Electronics > Beginners |
| MAX4460 inst. amplifier - maximum differential input voltage difference? |
| (1/1) |
| Ptzf:
Hello! I have a hall effect current sensor that outputs 185 mA per Ampere. The quiescent voltage is 2.5 V at 0 A, the current sense signal is summed op top of that. My ADC unfortunately demands a maximum of 2 volts at the input (0-2V that is). Now, my solution would be to use a differential amplifier to subtract 2.5 volts at the inverting input and set the gain to around 3.5 (3 A max input, thus (0.185×3)×3.5=1.94 V). I went for a simple solution (I can be corrected here) and tried out a MAX4460 instrumentation amplifier to achieve the desired goal. I set the gain with R2=68k and R1=33k, as specified in the datasheet. I fed the inverting input a steady 2.5 VDC from a power supply. The non-inverting input signal was also fed from a power supply. The following behavior was present with or without a load resistor at the output. Inst. amp supply voltage was 5 V. What I discovered was that the output of the inst. amp. was very nicely around 0.00V at non-iverting inputs below 2.5 V. As soon as the input went above 2.5V, I saw a nonzero output. What puzzled me, however, was that as soon as I reached an non-inverting input of 2.94 V, the output latched to 5V (the difference between the input ports at this point was 400 mV). A few millivolts below and the output went back to around the expected ~1,3 V. The datasheet says that this inst. amp is meant to amplify small diferential signals, and even specifies some ranges for other models than the MAX4460. My question: Is this latching behavior at "large input" voltage differences a "real life" property of opamps (inst. amps in specific)? I'm a opamp newb, so I have the feeling that I'm missing something. Or in general - whats up with this latching? |
| Wimberleytech:
--- Quote from: Ptzf on December 04, 2018, 01:53:35 pm --- My question: Is this latching behavior at "large input" voltage differences a "real life" property of opamps (inst. amps in specific)? I'm a opamp newb, so I have the feeling that I'm missing something. Or in general - whats up with this latching? --- End quote --- No. Generally it is not a "real life" property of opamps. Not sure what's up with the latching. Their input structure may be this: US6559720 https://patentimages.storage.googleapis.com/92/22/18/3f127117f41ba2/US6559720.pdf |
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