| Electronics > Beginners |
| Nanoamp current source troubleshooting |
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| ZeroResistance:
--- Quote from: exe on May 05, 2019, 08:04:25 pm ---Did you check on a real circuit, or in a simulator? --- End quote --- I checked in a real circuit. My next step is to connect a bipolar supply to it. Hopefully I won't run into any issues. |
| exe:
If it's a real circuit you can measure current and voltage drop on diodes, as well as opamp output voltage swing. |
| ZeroResistance:
--- Quote from: Kalvin on May 04, 2019, 01:41:38 pm ---In this particular case where using a battery, it is possible to introduce approx. 200 mV offset voltage between the signal ground and the negative power supply using a simple [schottky] diode. Without the offset voltage created by the diode, the rail-to-rail op amp will not be able to swing its output all the way down to the signal ground. However, introducing the diode, the negative power supply is now about 200 mV below the signal ground and the op amp is now able to swing its output down to signal ground potential without difficulty. Although the signal reference ground will have some ripple (60mV in this particular case) relative to the op amp's negative power supply even with the 10uF bypass capacitor, the typical power supply rejection ratio (PSRR) of an op amp will be able to take care of this "problem". Similarly, any temperature effects introduced by the diode will be attenuated by the op amp's PSRR. Br, Kalvin --- End quote --- I built the circuit as suggested. And then connected another 2 stages across, the load 1 a unity gain buffer and then a 100 gain non-inverting amp however at the final output I can see 50Khz noise, this noise seems to be riding on the dc signal, around 200mV p-p. My input is a 20mV dc signal. I was hoping to get some mains noise instead I end up with 50Khz noise. Circuit is powered by 6V battery and op-amp is LMC662 |
| exe:
--- Quote from: ZeroResistance on May 09, 2019, 01:17:23 pm ---I built the circuit as suggested. And then connected another 2 stages across, the load 1 a unity gain buffer and then a 100 gain non-inverting amp however at the final output I can see 50Khz noise, this noise seems to be riding on the dc signal, around 200mV p-p. My input is a 20mV dc signal. I was hoping to get some mains noise instead I end up with 50Khz noise. Circuit is powered by 6V battery and op-amp is LMC662 --- End quote --- Did you put compensation? Without compensation network it may not work stable as each element in feedback loop creates a delay. Delay == phase shift :). May be at 50KHz phase shift is 180 degrees. |
| ZeroResistance:
No I didn't put any compensation. Do I need to put in compensation for the 100 Gain amplifier? |
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