Op-amps, noise and high frequency/low current input signal

Click For Summary

Discussion Overview

The discussion revolves around the challenges of amplifying a low current (1nA) signal at high frequencies (around 1MHz) using operational amplifiers (op-amps). Participants explore methods to reduce noise in the setup while attempting to achieve a discernible output signal.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant notes that the current level is not critical as long as the source can drive a large load, suggesting the use of a large resistor (around 10 MOhm) to convert the current to a voltage for amplification.
  • Another participant proposes high-pass or band-pass filtering to reduce noise, specifically recommending filtering starting from around 300kHz.
  • A different participant mentions the input signal simulates particles hitting a plate at frequencies from 10kHz to 1MHz, indicating a constraint on filtering methods.
  • One participant suggests using multiple op-amps in series or parallel to further reduce noise, while also mentioning the use of decoupling capacitors and voltage dividers to mitigate noise from the function generator.
  • Another participant challenges the understanding of the signal, stating that it is not an AC signal but rather events arriving at certain rates, and emphasizes the need to distinguish valid events from noise for effective filtering.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the input signal and the appropriate methods for noise reduction. There is no consensus on the best approach to take, and the discussion remains unresolved regarding the optimal strategies for amplification and noise reduction.

Contextual Notes

Participants have not fully addressed the implications of the input signal being event-based rather than continuous, which may affect the filtering strategies discussed. Additionally, the effectiveness of proposed methods remains uncertain, as participants have not reached a definitive conclusion on their applicability.

GuitarOfWar
Messages
5
Reaction score
0
Hello everyone,

In my lab class we've been given task of using op-amps to amplify a current of 1nA and frequency of order of madnitude 1MHz to a discernable level. We've been given breadboards, a selection of different op amps, resistors, an oscilloscope etc to achieve this task. We've made good progress so far as we've gotten down to 10nA. But at 1nA we get to much noise to make a discernible waveform on the oscilloscope. Does anyone have any tips on how we may go about reducing noise in this setup?

Thanks for your time
 
Physics news on Phys.org
The level of the current is not very relevant as long as the source of that current can drive a relatively large load.

Since your desired output signal is a voltage, all you need to do is to pass the current through a large resistor and then use the op-amp to amplify the voltage across that resistor;. If you can use a resistor with a value of say 10 MOhm or so you will get a voltage any "normal" op-amp can handle.
That said, at 1nA you might need to use a few more "tricks". Since you know that the signal is at `1 MHz you can high-pass (or even better band-pass) filter the signal from say 300kHz or so, which will dramatically reduce the amount of noise

Also, this should probably be in the EE forum.
 
f95toli said:
The level of the current is not very relevant as long as the source of that current can drive a relatively large load.

Since your desired output signal is a voltage, all you need to do is to pass the current through a large resistor and then use the op-amp to amplify the voltage across that resistor;. If you can use a resistor with a value of say 10 MOhm or so you will get a voltage any "normal" op-amp can handle.
That said, at 1nA you might need to use a few more "tricks". Since you know that the signal is at `1 MHz you can high-pass (or even better band-pass) filter the signal from say 300kHz or so, which will dramatically reduce the amount of noise

Also, this should probably be in the EE forum.
f95toli said:
The level of the current is not very relevant as long as the source of that current can drive a relatively large load.

Since your desired output signal is a voltage, all you need to do is to pass the current through a large resistor and then use the op-amp to amplify the voltage across that resistor;. If you can use a resistor with a value of say 10 MOhm or so you will get a voltage any "normal" op-amp can handle.
That said, at 1nA you might need to use a few more "tricks". Since you know that the signal is at `1 MHz you can high-pass (or even better band-pass) filter the signal from say 300kHz or so, which will dramatically reduce the amount of noise

Also, this should probably be in the EE forum.
I can't bandpass the signal because the input is meant to be simulating particles hitting a plate at a frequency of say 10000kHz to 1MHz. I've put decoupling capacitors on the opamp so far. I was thinking of putting multiple opamps in stages in series. Would it be better to do it in paralell? What ever little tricks I can do to reduce the noise as much as possible. I usually put the input current through some voltage dividers to reduce the nouse the function generator makes at low ampltidues.
 
GuitarOfWar said:
the input is meant to be simulating particles hitting a plate

You have misstated the requirement. You do not have an AC signal. You have events arriving at certain rates. If you want to further reduce noise beyond a bandpass, think of an event detector, then count events per second.

What distinguishes a valid event from noise? If you can't answer that, you can't filter noise.
 

Similar threads

Op-amp circuits and noise at high frequency/low amplitude
  • · Replies 5 ·
Replies
5
Views
2K
Building RF Noise Generator Circuit w/ LM324D SMD Op Amp
  • · Replies 12 ·
Replies
12
Views
4K
Question about op amp input bias current
  • · Replies 8 ·
Replies
8
Views
4K
How Can I Improve Signal Transmission and Reduce Noise in High Magnetic Fields?
  • · Replies 2 ·
Replies
2
Views
3K