I am currently using a cmos single supply with 9 pA Ibias GBW 10Mhz low power battery powered app. I would like to find something similar that uses dual supplies. Do you have some recomendations for something that would work well given these requirements?
Thanks
MJ,
Most of TI's low power amplifiers fall under the single supply category. What is your application, specific power supply rails, and quiescent power target? Are you reverse-biasing the photodiode or operating it under a zero bias condition?
Matt
Hi Matt,
Thanks for the reply. have not worked with op amps much so much of this is new to me. I will be detecting signals with a si photodiode. I have been testing some using a single supply with anode to non-inverting, cathode to inverting, a feedback to give gain and some feed back capacitance to compensate for stability.
My main issue in using the single supply is that I am using multiple stages and using dc blocking capacitors in a couple of stages. I am using a first gain stage to amplify the signal an amount without saturating. This amplifies the DC, which i remove with a dc blocking capacitor. I then have to add a certain amount of DC back into the signal in order to keep it within operating rails to amplify in a the signal again in second stage. If i do this in 2 stages I have to have extra circuitry to add a small amount of DC back into the signal in order to operate correctly with a single supply amp.
It seem to me that that this could be simplified by using a dual supply amps. The signal would remain within the operating rails after the DC blocking stages. Would you recommend this approach?
As I have not worked with opamps much, I am not too familiar with what dual supply are capable of in terms of quiescent current. I would hope to keep it to a minimum. The rails would be +/- 3V. The main concern is to keep the ibias as low as possible. Are there any dual supplies that are fabricated using cmos?
Thanks for the help.
Your transimpedance configuration is a pretty standard photovoltaic bias and the typical key concern out of this critical stage is SNR (signal to noise). If you give me the SI photodiode you are using and your expected photodiode current range (i.e. 10nA to 10uA) I can recommend a single supply solution and a dual supply solution (including the gain stages). Keep in mind that all of the single supply OPA devices have a MAX supply specification of 5.5V so you can actually run them off +/-2.75V.
Do you have an ADC and a uC picked out for this application yet? At what frequency are you pulsing your photodiode?
I am looking at moving towards using a hamamatsu 6968-01 : http://sales.hamamatsu.com/index.php?id=13183125&language=1&undefined. We will be using them at a maximum distance from the source, so the current will be minmal, somwhere above the dark current > 5nA.
I am currently using a dsPIC. I am not sure if I will need a seperate ADC. The expected bandwidth is less than 100 khz.
Thanks.
What is your MAX swing range and levels that you will be feeding to your ADC? (i.e. 0-5V, 0-3V, -2.5 to 2.5V, etc.)
Matt,
At this stage I am not sure if i will be using an ADC. I hope to simplify processing by using a schmitt trigger with the threshold being set by a pwm output of the micro. I may need to look more closely at the analog data in the future and would likely need an ADC at this point. This being the case I would only be looking at 0-3Volts.
Since you are only looking for 0-3V out, I would recommend sticking with the single supply strategy as it stands the best chance at meeting your power and performance needs. Here is a possible starting point for you. If you truly need 100kHz bandwidth this pushes the power budget up a bit; however, if you can live with less bandwidth you can scale the OPA381 and OPA365 to something lower power such as an OPA364. FYI-- you will have to scale the feedback resistor, compensation cap, noise filter, and OPA365 gain stage accordingly. Note also that you should scale the reference voltage based on the output swing of the transimpedance amp, which in this case is the OPA381. This reference voltage can be a divider off the supply (if it is linear) with a low power buffer such as an opa333.
I hope this helps as a start.
Great information. Thanks for the help.