Looking for a tear down of DLPCA-200 - Page 1
Forum section: electronics / testgear

author: drinker
author_rank: Newbie
timestamp: April 15, 2015, 03:20:16 pm
content: Hi everyone!I'm in desperate need of a schematic for a DLPCA-200 Variable Gain Low Noise Current Amplifier (datasheet can be foundhere) - which I would like to emulate but in a lot more simplified form. It's basic use would be as a lock-in preamplifier of an antenna signal. It's a rather neat device with some useful functionality (as can be seen in the picture below)but in my case I would only restrict it to the following:GND or Bias of Input <- probably a better choice would be bias in case of weak signals to make a differential amplifier in the later stages (that's how I imagine this device works - what would be the point of biasing otherwise?)Offset <- i guess it's good to have that in the I/V stage (perhaps one could get rid of it in some clever way)Gain: 10^8 Low noise gain is the target -> also limiting the input to 7kHz is fineFBW ouputAC/DC at the end - not really sure what this is doing to be frankSo my question is if anyone has such a device and could provide a teardown? Another option would be to just design the thing from scratch given the aforementioned conditions - but I'm not sure I'm up to the task.I'd welcome any suggestions.Best regards,drinker

author: Marco
author_rank: Super Contributor
timestamp: April 15, 2015, 03:36:46 pm
content: Quote from: drinker on April 15, 2015, 03:20:16 pmAFAICS it's for providing power to the DUT mostly (in theory you could use it to minimize leakage for signals with DC offsets I guess, but you really want a driven shield for that). The example in the datasheet just connects a photodiode directly to the coax cable for instance.AC switches in a coupling cap after the transimpedance amplifier.Any way ... so you want a transimpedance amplifier with 10^8 V/A amplification, input bandwith 7 kHz and 13 fA/rtHz input referred current noise?

author: drinker
author_rank: Newbie
timestamp: April 15, 2015, 04:07:13 pm
content: Quote from: Marco on April 15, 2015, 03:36:46 pmWell in my case I have a photoconducting antenna that is being excited with a short laser pulse. Biasing it would, in my mind, also turn it into a emitter - in the best case offseting the input of the I/V stage.Quote from: Marco on April 15, 2015, 03:36:46 pmYeah, I noticed that after a few seconds of contemplating the datasheet - the placement of the AC/DC switch at the end threw me off. Anyway - is it advisable to keep the DC coupling if it's going to feed a lock-in amplifier working at a given frequency? Would it not be better to introduce at least a low frequency cut-off filter and get rid of some of the noise?Quote from: Marco on April 15, 2015, 03:36:46 pmGuess that comes down to it - yeah.

author: drinker
author_rank: Newbie
timestamp: April 16, 2015, 01:52:37 am
content: Well - I'm going to suggest a circuit that could perhaps do the job (let me know if I'm mistaken since I'm a total noob at designing or electronics in general for that mater - which is a shame really)I was considering two op amps for the job: OPA27 and LT1028. The component values for those two would be:CIN = 33nF (OPA27) or 270nF (LT1028)CF = 22pFRF = 1Mohm (seems huge)Which promises to give me a V/A ratio of approx. 1/CF = 5e7. So lets say I could bias the diode in the picture (antenna in my case) to give me an input current of around 1nA. Would I get a output current on the order of tens of mV?I know this is an extremely simplistic analysis - not considering the imperfections of a typical op amp - so I'd really welcome hints on how to include them in such a circuit. Also the DLPCA seems to have at least two cascaded amplifying stages - is that a necessity or is it optional?

author: Marco
author_rank: Super Contributor
timestamp: April 16, 2015, 05:48:37 am
content: They actually use 100 MOhm in that box for the low noise 10e8 setting. I'm not sure where the 50 Ohm input impedance for the device comes from though (for higher feedback resistors it's simply the feedback resistor divided by open loop gain, but that can't be it for the lower ranges). Probably input protection, which you can do without if you're careful.Cin is usually parasitic in a transimpedance amplifier and you'll want to keep it as small as possible (if you breadboard something do it deadbug).Dunno much about transimpedance amplifiers, but I do know that the required GBW grows with input capacitance and feedback resistance ... since both are rather extreme here (coax cable doesn't help) I'd try the OPA380, bit noisy but designed for transimpedance amplifiers, very fast and autozero.PS. I don't know what your time/money breakdown is buttheseare pretty decently priced ... throw a 100x amplifier after it and you have the 10e8 gain. Slightly noisier than using a 100 Megaohm resistor directly, but it might be sufficient.