Mosaic
Well-Known Member
Hi All:
I am using an LM358 as a unity buffer to act as a peak detector. The non inverting is fed by a 1n4148 diode and has a 0.1uf ceramic cap across it.
The oddity is, with no signal thru the diode ..the unity buffer output slowly ( about 5 or 6 seconds) floats up to max output (around 3.5V of a 5V supply). If I place a 10Meg ohm probe across the cap, it discharges in about 3 seconds and all is well, the peak detector works with the 10M load.
Ideally, it seems I might need a much bigger resistor ( 50M to 100M) to permit the peak to have any longevity. These are kinda expensive. I tried using a rev biased diode but the leakage isn't enough, the output still floats high.. A rev. biased schottky 1n5817 leaks like a sieve here...discharges in 0.25 sec. No luck with a rev. biased LED either...no load. Even an el cheapo 1 uf electrolytic..didn't leak enough to offset this weird effect.
I thought the OPamp required an input bias current. It seems this opamp has a -ve input bias current?
Since I am sampling every 1 mSec or so I can live with a bit of droop with a smaller resistor, but this effect is unexpected.
I suppose a 1uF cap might be the way to go....to offset the droop with a smaller/cheaper value 'load' resistor. With an equivalent resistance of about 150 ohms to simulate the OP amps output current of around 20mA avg I see a time constant of about 5xRC = .00015 Sec or .15 mSec. This is ok as I would have around a 0.5mS min charge time, with a similar reset time (less adc sample time of about 40uSec). 3 time constant charging gets the cap close to full voltage.
thx for any input.
I am using an LM358 as a unity buffer to act as a peak detector. The non inverting is fed by a 1n4148 diode and has a 0.1uf ceramic cap across it.
The oddity is, with no signal thru the diode ..the unity buffer output slowly ( about 5 or 6 seconds) floats up to max output (around 3.5V of a 5V supply). If I place a 10Meg ohm probe across the cap, it discharges in about 3 seconds and all is well, the peak detector works with the 10M load.
Ideally, it seems I might need a much bigger resistor ( 50M to 100M) to permit the peak to have any longevity. These are kinda expensive. I tried using a rev biased diode but the leakage isn't enough, the output still floats high.. A rev. biased schottky 1n5817 leaks like a sieve here...discharges in 0.25 sec. No luck with a rev. biased LED either...no load. Even an el cheapo 1 uf electrolytic..didn't leak enough to offset this weird effect.
I thought the OPamp required an input bias current. It seems this opamp has a -ve input bias current?
Since I am sampling every 1 mSec or so I can live with a bit of droop with a smaller resistor, but this effect is unexpected.
I suppose a 1uF cap might be the way to go....to offset the droop with a smaller/cheaper value 'load' resistor. With an equivalent resistance of about 150 ohms to simulate the OP amps output current of around 20mA avg I see a time constant of about 5xRC = .00015 Sec or .15 mSec. This is ok as I would have around a 0.5mS min charge time, with a similar reset time (less adc sample time of about 40uSec). 3 time constant charging gets the cap close to full voltage.
thx for any input.