High gain transimpedance amplifier

What is the value of Rin for the  LMP7721?

Hi Paul,

I'm not phisicist, I'm engineer, and whatever I answer here is not based on calculations, they're here just my "feelings".

Before you do anything, please don't forget to read Paul Grohe's articles:

http://www.edn.com/design/analog/4368681/Design-femtoampere-circuits-with-low-leakage-part-one

http://www.edn.com/design/analog/4375459/Design-femtoampere-circuits-with-low-leakage---Part-2--Component-selection

http://www.edn.com/design/analog/4395651/Design-femtoampere-circuits-with-low-leakage---Part-3--Low-current-design-techniques

A further reading is from Bob Pease: What's all this femtoampere stuff, anyhow? Google will find it.

 It is possible to change the transimpedance by changing the feedback resistor. But, since you will play in the sub-picoampere range, the switching element can be critical. I don't think that any semiconductor switches can give you the required performance due to their charge injection and leakage. Reed relays are somewhat better, but in the sub-pA world, the fewer the components you build in, the easier your life will be.

I wouldn't choose a SAR type ADC to digitize the signal. At such a low level, integrating type ADC's provide the best signal to noise ratio. If a dual slope converter is too slow, try a sigma-delta architecture, like ADS1251 or something similar.

Also, due to signal to noise ratio and due to accuracy, I wouldn't make the reference voltage variable. Maybe, I'm too conservative, but reference is reference. It should come from a dedicated voltage reference IC, must be stable and very well decoupled. A DAC introduces noise and inaccuracy. When you lower the reference voltage of an ADC, you also reduce the signal to noise ratio, the result can be that lower bits representing a true random number instead of your signal.

Isn't it possible to make the input light somewhat stronger? Some optical lenses, for example? Is it possible to keep the electronic in a cold chamber?

Don't thrust in any simulation software, always build a proto. Would you lead the input signal on an FR4 board? Than read the above articles once more.

Thanks Bela,

nice reading, this will help me a lot when I design the PCB itself. I need a good schematic now.

1) Regarding the multi-level gain, I also though that a switching element would add too much leakage to be usable. However, I found this circuit allowing different gain to be used. http://www.diy-electronic-projects.com/p212-Picoammeter-circuit-with-4-ranges What do you think?

2) For the ADC I take good note of what you say. For the signal to noise ratio I found out that with a Gain=1G, I could have a SNR=10 at 100Hz with 0.3pA. The AC transfer is -3db at 100Hz using a 1pF parallel capacitor. On the other hand, a Gain=10M would give me a SNR=10 at 7Hz only even if the AC transfer would allow a signal to get up to -3db at 2kHz.

This means that having different gain would help me have good SNR in the pA range at ~100Hz AND good SNR in the nA range at frequencies much higher. It also means that having a high resolution ADC won't be of any help for bandwidth improvements. I would have better SNR and bandwidth using different gain or is it me thinking wrong?

The input light, there is no way to make it stronger. We already use high quality optical components. However, we will need to be flexible on the frequency. I will try to increase the bandwidth as much as I can, but without compromising resolution.

Best Regards

I check the multi-level gain circuit I sent in the previous message and this is not good for transimpedance circuits.

Here is the circuit I am thinking of using.

Would the mechanical switch add too much noise? Or would another kind of switch be better?

If I put a BNC at the ouput in parallel to the ADC, would I add a lot of noise? I am thinking of using a DAC after the ADC for the BNC output.. what do you think?

Thanks Bela,

for the DIY schematic, it was in fact a waste of time, sorry about that!

About the 10GOhm amplification, I take good note of what you say. I will try and see what to expect with regards to SNR and bandwidth.

Thanks for the input on how to solder the reed relay, I wouldn't have though about it.

To clarify things on the output, I want to have a digital ouput, so I need an ADC. This way I think I may have more precision (when transferring the results to a uC)  than with a direct analog output connected directly to a BNC cable. However, I also need a BNC even if I loose a little precision (due to SNR) on that output, to give me the choice of using digital data or analog signal. For the BNC connector, you are right, a simple buffer would isolate it from the rest of the schematic, good idea.

I have 2 questions remaining:

1) I search for through hole resistor (10MOhm+) so I could solder them above a board in series to decrease their capacitance, but the only one I found where like 1 inch long(!!!)http://www.digikey.com/product-detail/en/SM104035006FE/SM104FE-500M-ND/824210 while the surface mount are 0805 type. Is there a kind of resistor that I missed that are shorter?

2) I used the low impedance node to drive the guard ring. I see that this is usually the Vref of the output. However, can I use the GND as the Vref of the guard ring or do I loose any effect when doing that? I also though to use a negative Vref (to have more range), but that would make the GND of the BNC connector negative which I don't think is good.

P.S. I bought the book, it seems nice.

Thanks a lot! Have a good holiday!

I will have the voltage changed to ±2.5V and remove the buffer amplifier!

I have another question! I found that reed relays can be sold with 50ohm coaxial shield. Is this something that could improve the noise resistance of the switch if connected to the guard shield?

http://www.cotorelay.com/index.php?option=com_content&view=article&catid=34%3Areed-relays&id=124%3A9900-reed-relay&Itemid=54

This Coto 9900 type relay seems to be a good choice. The 50 Ohm impedance plays a role in high frequencies. This reed are able to switch in the GHz range, so impedance there is important.

Since your transimpedance in the higher range is only 1MOhm, I don´t think, you will have too much of a problem. Even the 10GOhm range is not extreme, it is feasible. I do talk about DC precision and noise, not about AC bandwidth. You have to build a proto and test it for bandwidth yourself.

Good luck! (I hope you don't build something for weapons :-) )

lol no weapons no! 

I will be doing to proto very soon (after the holidays).

The high value resistor have a capacitance around 1pF.  http://www.ohmite.com/cat/res_minimox.pdf

Can I remove the 1pF capacitor in my schematic since it is already "generated" by the resistor?

Hi Paul,

this question cannot be answered through the internet. Build your first proto, and if it oscillates (which is, according to Mr Murphy, the normal behaviour of any newly built amplifier), than you can start to compensate it:

http://www.ti.com/analog/docs/litabsmultiplefilelist.tsp?literatureNumber=sboa055a&docCategoryId=1&familyId=1481

Although there are a lot of literature, how to compensate the transimpedance amplifier, they all lack of one thing: you cannot be sure, how much the photodiode's capacitance is, and how much is the amplifier's input capacitance is. There's something in the datasheet, but it varies with temperature, with bias voltage, varies piece-to-piece, manufacturer-to-manufacturer, so I say, drop in some capacitance parallel to the feedback resistor, test it again, try another diode, and do this until it works good, or you reach the deadline or until the working hours are all over :-) too much of feedback capacitance gives stability but reduces bandwidth.

When you think it's already stable, build a small series and test them piece to piece thoroughly.

With reference to the post by Bela giving a reference to the app note:

http://www.ti.com/analog/docs/litabsmultiplefilelist.tsp?literatureNumber=sboa055a&docCategoryId=1&familyId=1481

you may also find the following app notes useful: sboa060, sboa061 and sboa035. As the compensation feedback capacitor is often small and not well known it is possible to produce a variable value by resistively potting down the output signal and using a larger more available capacitor fed back from the potentiometer junction. This causes the effective capacity to be lowered by the potentiometer attenuation. This can then allow ready adjustment to give good and stable response. Since the required feedback capacitor is determined by the input capacity of the system (for stability against oscillation) then increasing the feedback resistor to achieve increased signal gain necessarily results in decreased bandwidth.

Scott Hamilton.

Hi,

I haven't had time to finish the circuit, but It is almost complete now. I am in the process of finishing the PCB routing. However, I have another question bugging me.

I made a guard ring and connected it at only one point using a 0ohm resistor to reduce current going through the guard ring. However, what do I do with the photodiode D1. It is connected on the high impedance node on one side and connected to ground on the other side. I looked http://www.edn.com/design/analog/4395651/2/Design-femtoampere-circuits-with-low-leakage---Part-3--Low-current-design-techniques and it seems that the other side is connected to the guard ring. My problem is that there will be some current going through that. So the question is, should I connect to ground or through guard ring? or maybe use this pin as the connection between GND and Guard?