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Noise or grounding problem in a two-stage Transimpedance amplifier circuit.

motmot

Hi all,

 

I have - what I think is - a grounding problem, or a noise problem generated somewhere in a two-stage Transimpedance amplifier circuit, I am attaching the schematic of the circuit, however, please note the following:

+ The two diodes you see "D1" and "D2" are only for the sake of graphical representation, in the circuit there are two Lateral-effect Photodiodes (or PSD) reversely biased.

+ What you do not see in the schematic are the four 20 kOhm potentiometers used for input bias voltage balancing of the 4 op-amps, if at all relevant.

 

I am also attaching the voltage measurements at some nodes in the circuit, sometimes under different connection conditions for reference.

 

The problem is also quite obvious when measurements are taken between different ground nodes (or what I am calling Common GND) where I get a kinda sinusoidal signal with Vp-p ranging from a 100 to 200 mV with frequency of about 30 MHz (an example is attached).

 

Note that the C_GND is not actually grounded externally, so it is relative, which might be the main problem. If it is indeed the problem, could someone point me to the relative best practice.

 

The result of what you see is the fluctuating output signals of the pair of op-amps at the second stage (also 1st stage has something similar) with frequencies of around 4 and 8 MHz -even with the 100 pF on the feedback of the op-amps- which should ideally be a DC signal. Therefore I would very much appreciate any comments or critics of the situation you see here to get me that sort of stable behavior.

 

Many thanks in advance.

Attachments:
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  • 0

    Hi motmot,

    A couple of things that have caused problems for me in situations like this:

    No.1 Failure to put 100 nF caps across the V+ and V-  on each chip.

    No.2 Poor connections and unwanted capacitive and inductive couplings.

    I would start with the caps on the chips.

    John

  • 0

    The LF357 has a GBW product of 20MHz and can manage to slew its output at 50V/us.

     

    Do you need that kind of performance? If not, it's a real liability given the form of construction you're using.

  • 0 in reply to D_Hersey

    Unfortunately, the PSD that was available to me had no reference to a datasheet, didn't have a part no., and I do not even know the manufacturer.

    If I remember correctly, with it hooked only to multimeters in photovoltaic mode it gave about a couple of microamps in the dark.

  • 0 in reply to jc2048

    The closed loop bandwidth needed for the system at the end is in the 10s of kHz, that's why I went with the fastest Op-amp available, as long as the other ratings are reasonable for the application at hand.

  • 0 in reply to jc2048

    Would you please elaborate on the liabilities of using an op amp with such ratings?
    By form of construction do you mean the breadboard type of construction? or the circuit layout itself?
    I'd appreciate more information, I am learning here after all image

  • 0 in reply to motmot

    At least, from what I know, having a high Opamp frequency response is likely to pick up more high frequency signals and amplify them, and thus what you are seeing might possibly be an oscillation of sorts which might not happen with an Opamp with lower GBP mainly because it's just physically incapable of producing such signals. These tend to be more picky about supply quality and need good bypassing to prevent oscillation as well. The amount of inductance and capacitance with breadboard traces probably provide just enough coupling to cause these sorts of oscillation.

     

    The other thing I can think of, having made a simple transimpedance amplifier myself (with ~10Hz response) is that the photodiode inputs should be as shielded from external noise as possible. External RF noise and impulse noise can get amplified, and most transimpedance amplifier design notes tend to recommend using a PCB and surrounding the photodiode traces with a ground guard ring to sink external stray fields.

     

    Unfortunately, aside from that, I can't claim to be any expert, so I'll let the others contribute their ideas.

     

    - Gough

  • 0

    Early op-amps had a capacitor to slow down the amp to stabilize it in low-gain situations.  The types are 'compensated.' stable at all frequencies, 'un-compensated,' and 'de-compensated,' which  was stable for gains greater than, like, five.  People were often too thick to use it, so I think it kinda fell from favor.

     

    How much dark signal do you get, what is the product of your leakage and your amplifier gain?

  • 0 in reply to motmot

    I'm not an expert in analogue design either.

     

    If you didn't need the performance, then it's a liability because it means the devices could oscillate. It also leaves them more susceptable to interference.

     

    You mentioned DC on the output, so it sounded like your system might have been very low frequency in its operation. In your case, however, you do need a reasonable performance since you've got signals up to several tens of kHz and if you drop the GBW product too far you won't have enough open-loop gain to drive the loop properly.

     

    By form of construction, I meant the breadboard. Your layout is neat and tidy and the layout is sensible, but for all that it can't be as good as a PCB.

     

    Just noticed, looking at the picture of the circuit board, that you could do with some bulk decoupling on your 15V and 5V rails. A 10uF or 22uF electrolytic (or tant) on each rail where the power comes on to the board would probably do.

     

    I don't understand the "Vbias...at the PS lead" and the "Vbias .. at diodes' common cathode lead" traces. One is a 531mV sinewave on 4.8V and the other is 1.62V. Aren't they effectively the same point, circuit-wise? And aren't they supplied by a 5V PSU? Nothing in the circuit is capable of driving the output of a PSU like that. Are you sure all the C_GND connections are actually connected together?