i am using a BurrBrown INA114 8pin dip op-amp (see attched pdf) to sense a current pulse across a current sense resistor. the problem i have now is that i dont see the oa's output as expected. i do see something, but i'll explain that later.
power comes from a single positive +5v regulator (lm7805) and its filtered well. pin 7 has two caps on it, a 10kpF poly and a smaller ceramic. the gain is set by a 1% 510ohm connected from pin 1 to pin 8. pins 4&5 tied to gnd. pins 2&3 are biased to gnd with 360k ohm resistors. pin 3 (V+in) is tied to the input of the resistor, and pin 2 (V-in) is tied to the output of the resistor. the load being switched on/off is basically a 5.6mH inductor. the flyback voltage is clamped at 34v via zener diodes.
the current flow is approx 1amp and is pulsed on/off 0-60Hz between 20-80% duty cycle. the gain of oa is set right at 100, so with 1amp flowing across the "curent sense" resistor i was expecting the oa output to almost reach supply voltage. the voltage drop across the resistor (as seen across pins 2&3) should be 1amp x 0.05ohm = 0.05v, and with gain of 100 the output should be 5v.
i was also expecting to see the shape of the output signal to follow along with the current flow. in other words, as long as current is flowing the oa output should stay high, etc.
what i observed look like noise. when the inductor current is switched off i was seeing very very short spikes with varying voltage up to about 3.4v.
its a very simple design, but is not working as i expected...
any suggestions or questions to lead me in the right direction.
thanks.
power comes from a single positive +5v regulator (lm7805) and its filtered well. pin 7 has two caps on it, a 10kpF poly and a smaller ceramic. the gain is set by a 1% 510ohm connected from pin 1 to pin 8. pins 4&5 tied to gnd. pins 2&3 are biased to gnd with 360k ohm resistors. pin 3 (V+in) is tied to the input of the resistor, and pin 2 (V-in) is tied to the output of the resistor. the load being switched on/off is basically a 5.6mH inductor. the flyback voltage is clamped at 34v via zener diodes.
the current flow is approx 1amp and is pulsed on/off 0-60Hz between 20-80% duty cycle. the gain of oa is set right at 100, so with 1amp flowing across the "curent sense" resistor i was expecting the oa output to almost reach supply voltage. the voltage drop across the resistor (as seen across pins 2&3) should be 1amp x 0.05ohm = 0.05v, and with gain of 100 the output should be 5v.
i was also expecting to see the shape of the output signal to follow along with the current flow. in other words, as long as current is flowing the oa output should stay high, etc.
what i observed look like noise. when the inductor current is switched off i was seeing very very short spikes with varying voltage up to about 3.4v.
its a very simple design, but is not working as i expected...
any suggestions or questions to lead me in the right direction.
thanks.
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