How Can I Achieve Low-Noise Amplification of 1nA to 1V?

[yungman]

What I meant is any HV circuit around in the vicinity. If your system don't use HV, you are ok.

You really don't have to measure, just get a few cap and try until you get one that give you the correct respond. Here is a 0.1pF:

http://www.newark.com/vishay-sprague/565r10gap10/capacitor-ceramic-0-1pf-1000v-z5u/dp/69K5436

http://www.johansondielectrics.com/leaded-capacitors/high-voltage-radial-leaded-capacitors/capacitance-and-voltage-selection.html

 

[Wapochief]

Have not had a chance to hook this up to the real source, but doing the calculation here:
http://www.analog.com/static/imported-files/tutorials/MT-059.pdf
It does not seem like the 200pf is a big deal. The feedback cap still needs to be very small... 0.0025 pf with a 100Mohm feedback resistor, and a larger feedback cap like we are using only makes it more stable. I think the 500MHz unity gain band width is helping out a lot here.

Edit: Ah I see its the 1+C1/C2 term...

 

[yungman]

0.0025pF is too small, you sneeze on it, it's more than 0.0025pF!

When I said HV, I meant over a few hundred volts, if you only have +/-15V, you are safe.

I am improvising here. If you have the big resistor close the the board, layout so that you put a copper strip that is connected to the output of the op-amp. The strip is placed right underneath the big resistor so it become a capacitor to the big resistor. Then you trim the strip until you get the response. Call me crazy, when you are disparate, you need to think way outside the box....way out! Try using a copper tape as the strip, start trimming and see whether you get the result.

 

[Wapochief]

I am still not sure how to calculate the effect of source capacitance. I read the MT-050 from Analog Devices, but it still does not make sense. Do you know of any other good resource for this?

 

[yungman]

I am still not sure how to calculate the effect of source capacitance. I read the MT-050 from Analog Devices, but it still does not make sense. Do you know of any other good resource for this?

In MT-059 you posted, look at Fig.1 and equation 1. That explained the noise gain due to C1 with is the input capacitance.

Did you try the copper tape trick to see whether you can get the low capacitance you need?

 

[Wapochief]

When I calculate c2 in MT-059, I get a very small capacitance (2*10^-14 f). According to MT-059, if I increase the capacitance, it increases the stability - makes sense since it dampens the gain. So I seem to be stable if I use my 0.13 pf for a linear amplification range.

In order to keep the noise low as described in MT-050, I have to calculate the 1+C1/C2 factor, which is around 1500 in my case. If I multiply it by the noise voltage of the op-amp, (7nV/rootHz) gives 1mV @ 10kHz - which is -59dBv.
This should be the dominating noise factor. Doesn't seem TERRIBLE, just a little high.

 

[yungman]

It is all about compromise. Speed you want is quite high so something has to give.

Why don't you do some calculation of using smaller feed back resistor and more gain on the second stage. Sometimes it is not obvious how it will go, you have to calculate it out. You are getting down to the theoretical limit, maybe you should write a quick Excel program and input different resistor value and post gain and compare the noise. I wrote one long time ago and I don't know where it is anymore, it was helpful. Just use the formulas in those MT for calculations.

Also, remember post #23 about two amp solution. If you can split the light, two amps afford you to use smaller PD with lower capacitance. That will really help your speed and noise gain. Not to mention the uncorrelated noise advantage on summing.

 

[Wapochief]

Just wanted to let you know the project worked like a charm! Couldn't have done it without you. We ended up using the OPA656, with a 100Mohm and a 0.2 pF capacitor. Ideally the capacitor should have been smaller to get better BW, but I couldn't find any. It was decided a gain of 10^8 was enough. About -90 dBv noise floor, 2 kHz of BW. It was enough. Surface mount worked well, just skipped the flux. Had to shield the board, grounding the shield to a separate ground to get rid of a large 60 Hz ripple.

Beat out the $$$ amp they were using.

 

[berkeman]

Just wanted to let you know the project worked like a charm! Couldn't have done it without you. We ended up using the OPA656, with a 100Mohm and a 0.2 pF capacitor. Ideally the capacitor should have been smaller to get better BW, but I couldn't find any. It was decided a gain of 10^8 was enough. About -90 dBv noise floor, 2 kHz of BW. It was enough. Surface mount worked well, just skipped the flux. Had to shield the board, grounding the shield to a separate ground to get rid of a large 60 Hz ripple.

Beat out the $$$ amp they were using.

Sweet!

 

[yungman]

Just wanted to let you know the project worked like a charm! Couldn't have done it without you. We ended up using the OPA656, with a 100Mohm and a 0.2 pF capacitor. Ideally the capacitor should have been smaller to get better BW, but I couldn't find any. It was decided a gain of 10^8 was enough. About -90 dBv noise floor, 2 kHz of BW. It was enough. Surface mount worked well, just skipped the flux. Had to shield the board, grounding the shield to a separate ground to get rid of a large 60 Hz ripple.

Beat out the $$$ amp they were using.

You make 100M surface mount resistor work without flux problem? Sweet. I never thought you can get away with this high value in surface mount! What resistor are you using, long body? Anything special you did?

Thanks for reporting back. People here have a habit of disappearing when they resolve the issue. You never know whether it work or not. When I ask a question and people help me, I always acknowledge what is the final outcome to close the chapter.

Alan

 

[Wapochief]

Just an 0805 100mohm surface mount resistor. No flux to avoid lowering the resistance. Didnt even cut out the pcb.

 

[yungman]

Just an 0805 100mohm surface mount resistor. No flux to avoid lowering the resistance. Didnt even cut out the pcb.

Now I am learning. How do you solder without flux? This is good information for me. I am doing some music electronics and I am worry about 100K resistors! Good to hear this.

 

[Wapochief]

Get a small tip, and put the solder on the tip. Hold the resistor down, then touch the solder tip to one end. Should flow across. Used a 1/32" tip at 350 degrees f.

 

[Wapochief]

Even a Gohm works if you don't use flux.

 

[yungman]

Thanks. What is Gohm?

 

[Wapochief]

Gigaohm or 10^9 ohms =P

 

[yungman]