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Nano-amp amplifier - 1nA to 1V

by Wapochief
Tags: amplifier, nanoamp
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yungman
#19
Feb24-12, 04:10 PM
P: 3,883
Here is another link by Skeptic2:

http://www.ti.com/lit/an/sboa061/sboa061.pdf

If you do decided to try 1G, use wires with thicker coating than the wire wrap wire for capacitance, that will lower the capacitance per turn. I don't worry too much about the 1G limiting the speed, it is your 200pF diode impedance that's the killer. Your noise gain is sky high for the speed you want.
Wapochief
#20
Feb24-12, 06:41 PM
P: 28
That design has a bandwidth of 30Hz. :'( I should have a test setup soon and I'll let you know what goes down.

I think I can lower the resistance noise by using a current divider in the feedback loop. Even then, I'll need a tiny capacitance across the feedback loop.
yungman
#21
Feb24-12, 07:32 PM
P: 3,883
Quote Quote by Wapochief View Post
That design has a bandwidth of 30Hz. :'( I should have a test setup soon and I'll let you know what goes down.

I think I can lower the resistance noise by using a current divider in the feedback loop. Even then, I'll need a tiny capacitance across the feedback loop.
Can you show me what you mean by current divider?

Are you stuck with the diode detector? Can you find a smaller one or you need the big area diode? 200pF is very high.

You can limit the BW to 30Hz? That would help.
Wapochief
#22
Feb24-12, 07:41 PM
P: 28
I still need at least 10 kHz...the design you showed me has only 30 Hz bandwidth. I am not using a photodiode....its for a "black box" that I can't change.

I found a way to "trick" the amplifier into thinking the feedback resistor is very large, when in fact it is small. On the output, put a resistive divider to ground. The top resistor is R1, the bottom is R2. Then place a resistor (R3) from the inverting to the middle of the resistive divider. The gain is about R1/R2*R3. You can place the capacitor across R3 to limit the BW.
yungman
#23
Feb24-12, 07:55 PM
P: 3,883
Quote Quote by Wapochief View Post
I still need at least 10 kHz...the design you showed me has only 30 Hz bandwidth. I am not using a photodiode....its for a "black box" that I can't change.

I found a way to "trick" the amplifier into thinking the feedback resistor is very large, when in fact it is small. On the output, put a resistive divider to ground. The top resistor is R1, the bottom is R2. Then place a resistor (R3) from the inverting to the middle of the resistive divider. The gain is about R1/R2*R3. You can place the capacitor across R3 to limit the BW.
No, not working, the voltage divider is the same as having gain on the second stage. No difference. Forget this one!!!! Sorry. I was thinking you might have a new trick.

Too bad it's a black box, I was even tempting to suggest making two circuits with smaller diode for detection. Each detect half, you add two, you get back the voltage gain, but the noise is root mean square and is not double. Any possibility of using two transimpedance amp in parallel and sum later?

I don't recall I show you a circuit that only has 30Hz BW!!! We always talked about the 10KHz and that's where the problem lies. 10KHz and a 200pF input capacitance don't agree!!!
Wapochief
#24
Feb24-12, 08:06 PM
P: 28
That would work, but we would need very good linearity. Seems like I would need a LOT of amplifiers to make a difference. The latest link has 30Hz BW.
yungman
#25
Feb25-12, 03:47 PM
P: 3,883
Quote Quote by Wapochief View Post
That would work, but we would need very good linearity. Seems like I would need a LOT of amplifiers to make a difference. The latest link has 30Hz BW.
When you get down to the limit, who said it would be easy. If you can double the signal with 1.41 increase of noise, that's a big gain!!! If you can do it, don't rule this out. Doing nitrogen is more complicated.
Wapochief
#26
Feb28-12, 01:58 PM
P: 28
The 100MOhm resistor worked, with the wire wrap I am seeing over 200khz of bandwidth. The noise floor is -70db, and amplifying it by 10 will make it about -50db - not what I need, but close. Overall a good success. Thanks for your help.
yungman
#27
Feb28-12, 02:28 PM
P: 3,883
I am glad it all work out. The wire wrap wire is nice!!!

Alan
Wapochief
#28
Mar1-12, 10:28 AM
P: 28
Any idea where I can get a variable cap that small? (0.1 pf or less) The customer does not like the wire wrap. Couldn't find anything below 0.25 pf and I need around 0.1 pf.
yungman
#29
Mar1-12, 01:30 PM
P: 3,883
You might have to use one fix and a variable in series. But the question is whether you have any high voltage around?

It is important that there is no high voltage around, If you have two cap in series, the middle floats. In the presence of HV, the open junction will charge up and arc and will burn electronics. This is very important.

If you have HV, you have to use two 50M in series, then you can put cap across each resistor. Now you can use bigger cap. One fixed and one variable.................Do you really need a variable cap if you can do the experiment to find the correct value. This value should not change in different amplifiers.
Wapochief
#30
Mar1-12, 05:51 PM
P: 28
I am not sure how to measure the capacitance, since it is so small... We are using +/-4 and +/-15 volts supply, and the output is only 1v maximum(with 10nA input). I would assume neither is high voltage.
yungman
#31
Mar1-12, 06:33 PM
P: 3,883
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...z5u/dp/69K5436

http://www.johansondielectrics.com/l...selection.html
Wapochief
#32
Mar8-12, 10:02 PM
P: 28
Have not had a chance to hook this up to the real source, but doing the calculation here:
http://www.analog.com/static/importe...als/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
#33
Mar8-12, 10:11 PM
P: 3,883
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
#34
Mar9-12, 09:03 AM
P: 28
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
#35
Mar9-12, 12:35 PM
P: 3,883
Quote Quote by Wapochief View Post
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
#36
Mar9-12, 01:24 PM
P: 28
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.


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