Measure very small changes in photodiode nanoamp - Page 1
Forum section: electronics / projects

author: KenthJohan
author_rank: Newbie
timestamp: January 14, 2022, 11:23:33 am
content: Hi, I'm going to do a experiment which is analyzing a reflection from a constant light source.I will try to plot the light intensity from a photo-diode.The problem is that the photodiode variation is in nano ampere and will have a offset from a constant light source. So like 2300nA +/- 100nA.I want to remove the offset and then amplify the variation so I can have more resolution of the variation range for the ADC.I'm also thinking about adding a DAC to control the offset and maybe controlling the gain using a digital potentiometer?Then it will be truly generic measurement instrument if I can configure offset and gain using microcontroller software.Note that I'm not good with electronics but I'm good with programming. There are so many op amp to choose from. I might use the LM324 but I feel like its crappy because its cheap. I also don't want to waste money on expensive op amp if its unnecessary. I think there is a delay in the LM324 when I'm simulating it, which make me believe its a slow op amp.Its mostly the noise that I want to minimize the delay doesn't matter to much.So I guess I want a low noise, any speed op amp.The electronics will be powered by batteries because battery is low noise.I will try to design and order a PCB when I'm comfortable this might work.I'm interesting in your thoughts about this.This is what I have come up with currently that might be able to measure very small changes in photodiode.http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWEBmAHAJmgdgGzoRmACzICcpkORIW1SJICApgLRhgBQA7iOluiDY5e-EFSgcATuByRBYYXwFCo4eHA4BLGapalhcuW1jrTpsKywcA8iIHilIVHIiRuMowo8giqCQHNBfV4iI2D8AwltL18g4VjDE3h0ZHc9SPSfPzceGL8vQ3c87wSpH3EVBFx5SLBTdyrhSurSniIKmPFkATdbRqyxUIHXNODYzPQhnLiQuVjJwoAPQSw-FmQIZXQ-ai8ABQALAHsAFyOAE00Lpg5lhAZaGnQ5LCwfHWsAVxOAB2+AHQAzmcgQBBAAiAGEgZAAG5AkGA5DQZCwspoagqdqYgpqDShUjeByicSuaCRGCQFJ1MxmAToDgE8rUWL9WJkimwam0um8dzPPzhSB+BYSHgIMACZyMSWzCQAJRlAi8Ep6qjmciQhig0AQHEVApQ9OFOm1Qy16t1HECGJqKFQuwp-JJLPE4mmhtFqrlbiZ7Nk4CIjpcOs5VI2PNM9P1IGCksFwnYao1jHVOr1sNjidIAnCU1UQ2RyDTMD1PDzck9UzKxGo0ti0pc8DKDaM3Sc2ppGjaIqGFbFA3r+SDElste847A1RGivWUrbytlZsr0Cn2UtZcDdZcI4UhVyI+le47A-ZLV7S0EqGLYGQwmQuzI7y81gAZq-AUwTkDX5IjgBbIFwVBSEODQXgQAAMVceQkkjeALBYN4wDg+DIEQt42BASFPkkSQmAAOxOMC-GQN5oLTIt5BAAA1I4ABsTgAQ38G4gA

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2022, 11:44:23 am
content: The LM324 is not ideal, as it has quite some noise (especiall current noise) and has a limited output range. On the other hand 2 µA and 100 nA change in the current are not yet really large so a very senstive amplifier is not really needed. A better (and still not that expensive) choice would be something like MCP6002 or TLC272.The TIA should use a larger resistor in the feedback and less gain after that. The noise if the resistor gets important when the voltage at the resisor falls below some 50 mV (than the shot noise and resistor noise get comparable).It is not ideal to move the positive input of the TIA, as this also changes the voltage at the photodiode and also lowers the useful output range. So better add the shift at the input of the 2nd OP. For low intensity and if speed is not imporant near zero voltage for the photodiode is the suitable operating point.A DAC to shift the DC level can work, but one still needs to look at the DAC noise there. So not every DAC is good and some filtering may be a good idea.Digi pots are a possible suitable type of low resolution DAC with resonable low noise. An alternative to DAC and than likely a limited resolution µC internal ADC may be the use of a higher resolution external ADC (e.g. MCP3421, LTC2400,...).If possible I would modulate the light to detect (e.g. use a LED to turn on and off regularly). This would than allow to use AC coupling instead of the DAC. Chances are also that the natural variations in the background are smaller at the modulation frequency than for DC.

author: KenthJohan
author_rank: Newbie
timestamp: January 14, 2022, 03:13:45 pm
content: QuoteI wanted to map +/- 100nA to a integer in microcontroller sosmallest_change = 200nA / (ADC resolution).that is detect if its changing ~1nAQuoteThat sounds interesting. I will try this if I can, might be too advanced for me.I guess I have to use a 555 timer for the LED and a ADC that can measure frequency maybe and a high speed OP. I'm not sure yet. I will look into how to do AC coupling.QuoteLTC2400 is 24bit that's much better! I completely ignored the fact that there exist high resolution ADC. I was thinking about using the 10bit ADC in the Arduino.24bit make smallest change really small:3000nA/(2^24) = 0.00017881393nAQuoteIf DAC has too much noise maybe I will use a potentiometer instead. And then I could possible attach a motor on it later if I wanted.Thank you for your suggestions.It seems that using only one OP and a high resolution ADC is the best option. This simplifies a lot.

author: jonpaul
author_rank: Super Contributor
timestamp: January 14, 2022, 03:49:17 pm
content: Photodiode amps and current noise is very well documented.Depending on the device specs, the noise an DC leakage can be calculated from the diode specs.Use of a large area lab grad diode form EGG, UDT, MilleGriot gives best results.Finally light sources in general are notoriously unstable and a fixed light measurement within 5..10% is considered very good.What is your intention and experimental setup?Exactly what light source, optics and reflection ?A sketch of the optical setup will be helpful.Bon Chance,Jon

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2022, 04:17:40 pm
content: Arificial light usually has a lot of 100/120 Hz modulation. So this light is not a smooth background, but with lots of ripple.For the LTC2400 and similar 24 bit ADCs the resolution is often noise limited and usually are not getting the full resolution noise free. The nice thing with some of them (and the right setting to a relatively slow speed) is that they can suppress the mains frequency related hum by quite a bit. The µC internal ADC my run into problems with this, as the hum may exceed the 200 nA of signal.100 pA resolution is not yet a very sensitive range that would requite a special lab grade photodiode and likely no special OP (unless the sensor is very large).Modulation gets especially effective, if the µC also knows (and maybe controlls) the modulation. Than one can than use a syncronous detection (like a lock in amplifer) and this way get good background suppression.

author: TimFox
author_rank: Super Contributor
timestamp: January 14, 2022, 04:19:17 pm
content: One good way to reject ambient light from the photocell current due to your light source is to modulate the light source at a fixed frequency (not harmonically related to your line frequency) and pass the photocell through an AC-coupled amplifier to a phase-sensitive detector driven from the light-source modulator's frequency.

author: jmelson
author_rank: Super Contributor
timestamp: January 14, 2022, 04:27:19 pm
content: Your circuit seems to be applying 218 mV bias to the photodiode. They work MUCH better when reverse biased at at least a few Volts. Noise and leakage currents actually drop when the junction is depleted, and capacitance also drops, which helps reduce noise gain in the amplifier.Jon

author: Picuino
author_rank: Super Contributor
timestamp: January 14, 2022, 05:55:53 pm
content: Lock-in amplifiers work very well for this type of problem.https://en.wikipedia.org/wiki/Lock-in_amplifier

author: Picuino
author_rank: Super Contributor
timestamp: January 14, 2022, 06:12:21 pm
content: Lock-in amplifiers may seem a bit complicated, but once the theory is understood, the practical application can be straightforward. Here's an example of a simple milliohm meter based on that principle:http://cappels.org/dproj/dlmom/dlmom.htmlEdit:https://physicsopenlab.org/2019/08/20/lock-in-amplifier/https://physlab.org/wp-content/uploads/2016/04/Sultan_presentaion.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2022, 07:30:42 pm
content: Loch in amplifier sounds complicated, but the principle is simple: With a modulated light measure the difference between the times with and without the light and repeat that over man periode. This can be done analog with an analog mixer and low pass filter / integrator or digitally by ADC conversion of the individual steps and possibly averaging within the steps.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 14, 2022, 07:56:30 pm
content: The usual approach is to use mechanical shutter and a lock-in amplifier, like in this demo at the end of the video, at about minute 32:00If you don't have a lock-in amplifier, you might be able to improvise one from an oscilloscope, like this:https://www.eevblog.com/forum/projects/oscilloscope-with-trace-averaging-as-a-lock-in-amplifier-(rigol-ds1054z)/If you still don't have either an industrial lock-in amplifier or an oscilloscope with averaging, you can try building a simple lock-in amplifier by your own design or by searching for a ready made schematic.Pick a simple one that is suitable for your type and ranges of measurements only. It doesn't has to be a generic lock-in amplifier that are meant to deal with almost anything. Those can get very complicated to DIY, and are very hard to design for such a wide spectrum of performances. You only need a simple one that will work reasonably OK for your particular kind of measurement.

author: TimFox
author_rank: Super Contributor
timestamp: January 14, 2022, 08:00:52 pm
content: Many industrial electro-optical devices, such as interrupters (with a slot between the light emitter and photosensor through which an opaque vane passes) have very simple synchronous detection of the photodiode current in order to reject ambient light: the circuitry fits neatly inside the package.One example:https://www.hamamatsu.com/resources/pdf/ssd/light_pic_kpic9001e.pdf

author: KenthJohan
author_rank: Newbie
timestamp: January 14, 2022, 09:14:08 pm
content: I can't use expensive bulky instrument I need to build many smaller ones that does not cost like over 1k $.QuoteI won't say the final intention. But I can say the intention is to see when some kind of fluid changes reflective properties when additives are added.LED shines on fluid -> fluid reflect -> photodiode receives light. The reflection changes over time under 10 minutes and I will log all of it in a microcontroller.I'm not into chemicals stuff so I have no idea how it works. I'm doing this for a chemist who also probably doesn't what he's doing.I don't know which LED or photodiode he's using. Its some kind of special homemade or secret, All I'm allowed to know is that LED consumes some mA and photodiode has variation in 2200..2400nA but this range might change also. Personally I would like to check for much smaller changes because that seems like its not that hard to do.QuoteI know that LED temperature might be a problem and I could probably compensate that in the software side.QuoteThe light source is a LED. The reflection comes from some kind of fluid that changes reflection depending on what it contains.No optics. It will be isolated in a box from external light sources and global illumination.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2022, 10:04:45 pm
content: With a likely closed system it is a bit unusual to have so much DC background.2 µA of current looks pretty high for a photodiode. So this may be a rather special photodiode (e.g. IR range). The suitable votlage depends on the photodiode. Some like a negative bias and some work better without.With the option to control the LED(s) it is definitely a good idea to preriodically turn on / off the LED at a reasonable fast speed (e.g. some 200 Hz range).Going up with the frequency helps against 1/f noise.This allows easy AC coupling of the signal and thus suppress most of the background. For synchronous detenction there are than 2 options:One is a hardware mixer (not very much effort, could be some CMOS switches like 4053) and than read the filtered part.The other option is to digitize the signal and than do the syncronous dection numerical by areraging and taking the difference.For the dirft of the LED, one may have a 2nd channel with a 2nd photodiode. At least the more normal Si photodiode are cheap and quite stable in the response, much more stable than the usual LED.The forward voltage of the LED could be used to get an estimate of the LED temperature drift.

author: ejeffrey
author_rank: Super Contributor
timestamp: January 15, 2022, 03:20:33 am
content: Quote from: KenthJohan on January 14, 2022, 11:23:33 amThis is a classic application of lock in detection. If you can find a way to chop your desired signal at a specific frequency then do synchronous detection of the photocurrent you can eliminate background signals including slow drift and 120 Hz flicker. The tradeoff is that the detection bandwidth is usually only a small fraction of the modulation frequency. If your system is amenable to this it is a very robust way to get good SNR.

author: Picuino
author_rank: Super Contributor
timestamp: January 15, 2022, 12:43:54 pm
content: Another application note about digital lock-in amplifiersImplementing Digital Lock-In Amplifiers Using the dsPIC DSC:https://ww1.microchip.com/downloads/en/Appnotes/01115A.pdf

author: TurboTom
author_rank: Super Contributor
timestamp: January 15, 2022, 12:53:00 pm
content: I'ld say you've got to look at your sensor configuration. Modulating the LED won't help much since this will also modulate both the "common" signal as well as the "effect" signal. Modulation of the source only makes sense if the "common" signal stays unmodulated (for example, look at the optical sensing of the "dip" in a rubidium atomic clock -- here the signal situation is similarly severe, but it's possible to modulate the "effect" independent of the signal).Since you cannot disclose what kind the sensing problem exactly is, all that we can guess and advise is what Kleinstein already mentioned: If possible at all, have the same LED (and not a second, identical one) illuminate a second, reference measurement cell which contains a reference fluid and possibly wire the two photodiodes directly in series, so at the common point, most of the "common" signal cancels out and what's left contains an effect signal that's at least two orders of magnitude cleaner, and that's much less dependent of variations in the light source.I'ld also check if possibly other (optical) effects may augment the "effect" signal, i.e. try polarization filters and place source and detector at an angle or the like. Maybe you can multiplex between two wavelengths that how different effect. If optical density changes, you may try experimenting with looking at interference patterns (though this may be too expenive since it requires high accuracy optics). But laser diodes are quite inexpensive and you may have a prismatic measurement setup and measure the deflection of the beam. It may even be possible to use completely different sensing techniques that may prove more effective like dielectric constant / dissipation, electrical conductivity, viscosity or speed of sound (ultrasonic sensors), thermal capacity...It's surely possible to make a laboratory grade one-of sensing system according to your requirements (that probably needs frequent re-calibration), but a (high) quantity production run that's sufficiently stable to work over a prolonged time "in the field" is highly questionable IMHO with the proposed signal properties.

author: nfmax
author_rank: Super Contributor
timestamp: January 15, 2022, 09:31:50 pm
content: This sounds to me like the sort of situation I would step quickly away from, gracefully declining to get involved. If your friend/colleague was wanting help with a one-off experimental rig, that would be different, but a series production run based on his secret optics hardware, which you are not allowed to question or even know much about, is a recipe for disaster. Itwontwork 'properly' (whatever that may be) andyouwill be blamed.Get a proper NDA in place, and get him to explain to you what he is trying to do and how he is doing it. Then go read Phil Hobbs's bookBuilding Electro-Optical Systems: making it all work, take expert advice where necessary and come back here with any remaining questions. There are a number of us on the board with electro-optics experience who may be able to help.Don't commit yourself to anything you may regret later.

author: Kasper
author_rank: Frequent Contributor
timestamp: January 15, 2022, 09:54:50 pm
content: Quote from: KenthJohan on January 14, 2022, 09:14:08 pmSounds similar to PPG optical heart rate sensors.Look up PPG AFE (analog front end). There are lots of ICs and dev kits to hold the ICs. Some of the AFEs come with LEDs or LED drivers, PDs, op amps, filters, etc.Search sparkfun for PPG, they should have a dev kit for this.

author: Someone
author_rank: Super Contributor
timestamp: January 15, 2022, 10:05:28 pm
content: Quote from: TurboTom on January 15, 2022, 12:53:00 pmThere are routine and robust (used/installed by non specialists in outdoor and/or portable applications) optical measurement systems out there that hit the sort of electrical performance the OP is asking about and easily retain stability/reproducibility/accuracy under 0.01% of the common mode "offset".But the people who know have to make them arent going to give the details away for free.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 16, 2022, 01:53:02 pm
content: I dont know how you your light source works but I assume its is not in any way modulated.Read up on lock in techniques- try and borrow a lock in amp. This is usually the way to go.seehttps://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjMsOrEsrb1AhVeQEEAHXftAasQFnoECAIQAQ&url=http%3A%2F%2Fwww.chem.ucla.edu%2F~craigim%2Fpdfmanuals%2Fappnotes%2Fan1003&usg=AOvVaw3_DCJFJTmFxToZnOUCNgAKDIY lock in amp AD630.One very common method is to use an integrator to servo the ambient light component down to near zero. Set the intergator time constant to be at least 10x lower than your changes in reflected light. It's a high pass filter in effect.https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/182579/photodiode-ac-amplifierhttps://www.edn.com/photosensing-with-ambient-background/The downside is settling time after switch on or big cnahedsin ambient light- you can add an inegrator reset to discharge the cap if you detect an overload.An other strategy is to store the ambient light voltage on an S&H and periodicaly use it to remove the ambient offset. change.You can use a digital "infinite S&H" made from an ADC and a DAC Using the S&H approch lets you meaure down very slowly changing events.https://www.analog.com/en/technical-articles/infinite-sample-and-hold-outperforms-many-legacy-sample-and-hold-amplifiers.htmlori/o ports on a uC if it can give you enough resolution. TI make a photodiode am and S&H in one chip: OPA1S2384/ OPA1S2385. All you need as good wideband precsion opamp to complete fb scheme above. For the amplifier use a design tool like ADshttps://tools.analog.com/en/photodiode/orhttps://www.ti.com/lit/pdf/sboa055Be aware that input capcitance at the amplifier will cause oscillation if not properly compensated for. Avoid long leads between the diode and the amplifer.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 16, 2022, 02:04:10 pm
content: The DC servo circuit is just a different way to implement a high pass filter. So it replaces AC filtering with not too much advantages.Adding current directly to the TIA input is nor very helpfull, as it would cause more noise from the current noise. So a simple TIA scaled to handle to DC current is good enough, simpler and generally lower noise.A DIY syncronous detection does not need an expensive AD630. Especially with simple on/off modulation a switching version with CMOS switches like 4053 is often better performance.If the modulation also effects the background part depends on the unknown details of the photodiode and optical system.

author: KenthJohan
author_rank: Newbie
timestamp: January 17, 2022, 04:00:29 pm
content: I'm going to try both photoconductive and photovoltaic mode.The PCB will have 8 channels. The MCP3421 support 8 I2C addresses.I will try to make it generic so its not too much tailored.KiCad already had MCP6002 and MCP3421 so no custom footprint needed.Future iterations will include nRF52840 Dongle for Bluetooth communication.Later I will release the kicad design files and the source code which will depend on zephyr project, public license.I'm going to keep posting the progress here for fun.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 17, 2022, 06:19:51 pm
content: Photovoltaic mode is slower and inherently non linear. Vo~log*Irradiance. The amplifier designs need careful attention to detail- the basic textbook circuits will probably oscillate wildly. The input capacitance needs to be tamed with correct compensation.Do as much signal processing as possible before digitising. Garbage in garbage out.Consider making a pair of light to frequency converters. To achieve this connect the output your PD amp into an AD7740 for intance. Either stand alone or in a pair as described below. It's then just a simple counting / pulse width measurment job for the uC. Very high resolutions are posible using this method.Top tip. LEDs can be used as super cheap photodiodes. Their response is wavelength dependent- ie a red LED detects red light with the response curve as shown in the LED datasheet.Two adjacent surface mount LEDs from the same cardboard strip are likely to be very similar in their spec.Tune up the gains of the amps so cancel out the steady state light. Even if the null is not perfect you'll still get a lot more bits of resolution.You may (should) want to use a bandpass filter to narrow down the frequency range.Can you arrange for the burried signal to be at a different wavelength to the source?If you can, this will create even more measurement band narrowing. Always good for SNR.I think the labels for the amp inputs are the wrong way round. You need an antialaising low pass or bandpass filter on the ADC input and some protection if you run the opamps off 5V.The max input fault current for the ADC is 2mA so R? needs to be greater than 2.5K. The datasheet suggests using 3K. If you can, run the opamps of the same supply voltage as the ADC. One less power rail and one dead ADC to worry about.

author: KenthJohan
author_rank: Newbie
timestamp: January 17, 2022, 06:38:52 pm
content: QuoteI don't know if I should get an extra battery for negative voltage or not yet. I think it can be done without negative voltage.http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWEBmAHAJmgdgGzoRmACzICcpkORIW1SJICApgLRhgBQAluLiG2HS8c-IqihRY8SOkhEOAeRBEq-ZEJXUwIiJA5FIpFHLUbtp8JJGRJM5GGmPpQ9BwDmw8IM+DxNvQBKKAhCAkLIIfza-hIGjBL+0AgcAO7BoaoRoexCemlZUd5gfGFQHABOnmw6fDkSDtKp6RIFdXnNdcU63u1dUSLIJtVl+UNiIOhYoeN6lUTmLOrK5ks2DfAVywNm3bngjWmT0+JHKLlNp0vz2yNbFpoWekp9w6Qiw7pNrd7XXucARiBUDZqjZ0OD+OgjHoAB5A8RsQYTShRSbKCbUAAOAAsAPYAF1xABMuMSmE0XuMXuY9ETwOgEVSGYUXCAiUwAGYAQwArgAbfEU5ksKklGZChGdEptDhoCQAMV0CSk8GQURAAGEeeVykwAHaCoA

author: TimFox
author_rank: Super Contributor
timestamp: January 17, 2022, 06:41:13 pm
content: "Photovoltaic mode" for a photodiode in a detector (not solar power) usually means with zero bias applied, as opposed to "photoconductive mode" with an applied reverse bias.Many detector systems use zero-bias photodiodes driving a virtual ground current-to-voltage amplifier input to get a linear response to the light input (voltage at the amplifier output) without the temperature drift of the diode's dark current. The resistance (inverse slope ofIvsV) through the origin is finite, and generates thermal noise like other resistors, but there is no DC leakage current (except for that resulting from the finite voltage offset of the op amp. An important problem with photovoltaic mode is that the shunt capacitance at zero bias is quite large, compared with that of a reverse-biased diode.

author: KenthJohan
author_rank: Newbie
timestamp: January 31, 2022, 03:17:24 pm
content: Now I'm thinking about using both DC and AC coupling. I figured out a way to capture both. This is photo-conductive mode with 3.3v reverse bias.http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWEBmAHAJmgdgGzoRmACzICcpkORIW1SJICApgLRhgBQA7iOluiDY5e-EFSgcATuByRBYYXwFCo4ePG4y5KhXKKoJAcxEDkOA0pBE4qyBwCW84S2TKFVg3JjrIYdBR9A3w4AeRMxamQzCPAJHl0QVDkEv08OAA9eVAMiASIwHPoZEABBAGEAHQBnEIBXABcABwbq+oB7apKAETKOABMPQf1BV14QPqYAMwBDWoAbes0VYZW0nhUXPINNiWtSLRjLcQgYYS9YdGQwIMCBdE0U9AtIA1S43heQUkVPt7sAJQ+BlG6E+XlU1kYtig0AQHAAxgdVoNkqx9iwYH5IEQsN9sWZ0PpIKRPLBgoDhjtKUloZDkNBkNCYHC9gdCXJLOzYqdbBcrjcfHcOIDQc8xeBcLS5EhwcypOEuZZkNjVNd1PKlSqleZVeqeJrqOxTCq7PFZLxCeA-BbqKarXdLUaUCbNKKFZ8zGtwp6gSgdXbhj63T67MZHq9zQkvJphvhhAk4xIwpy6dEuScMll9ugwPtchz0Hlij1qnUmi0qu1Oj1QoNEwXDRJpLGEMJA-61HqhjkLK2JGgrCAAGInGE+NUC9QCbxTyc+RlsEBlWqSSRMAB2iyAA

author: tggzzz
author_rank: Super Contributor
timestamp: January 31, 2022, 03:30:44 pm
content: Quote from: TimFox on January 14, 2022, 04:19:17 pmYup.I used a similar technique in 1979, and using a new-fangled LF351 was able to detect pW changes.The OP should reverse-bias a PIN diode plus transimpedance amplifier to ensure linearity.

author: KenthJohan
author_rank: Newbie
timestamp: February 07, 2022, 04:10:18 pm
content: I have discovered that some ADC lets you adjust voltage offset with VREF- and VREF+.I'm planning on using theMCP356124bit 8 mux channel ADC.* Optional trim pot can adjust VREF- and VREF+.* Optional resistor divider can adjust VREF- and VREF+.* The PCB will support DC exclusive or AC coupling mode with optional capacitor.* A optional 10ohm resistor is available on every power input for a weak over current protection.* Every power input has a bypass capacitor.There is a lot of resistors and capacitors so I will have JLCPCB solder them for me.PCB drawing is not finished yet

author: Kleinstein
author_rank: Super Contributor
timestamp: February 07, 2022, 08:39:02 pm
content: The MCP3561 is the 2 channel version. The 8 channel one would be the MCP3564.I very much doubt one would need fine adjust for the ADC reference. Fine gain adjust would be in math - especially with 8 channels using the same reference.The resistor to ground at the TIA inputs is no needed. It would only add to the noise.As shown with the + input connected to ground / neg supply, the TIAs would work with negative current only. This could be a problem with an AC coupled input, as AC by design needs both signs. So one may wand to raise the + input level to something like 1/2 the reference. At last make it accessible to adjust to a convenient level.

author: KenthJohan
author_rank: Newbie
timestamp: February 10, 2022, 04:00:43 pm
content: Good advice! Yes I meant MCP3564. I was reading MCP3561/2/4 datasheet where I copied the name.Updates:* Removed ADC reference trim pots* Removed the resistor to ground at the TIA* Added voltage divider on each op-amp+ for AC coupled mode to measure both signs.Not sure if yet if I should let JLCPCB solder the components.I only have to test one channel first to see if it works before I solder everything.Now I have to find where to buy components, seems to be a chip shortage now.* ADChttps://eu.mouser.com/ProductDetail/Microchip-Technology/MCP3564R-E-ST?qs=sGAEpiMZZMv0NwlthflBi0CMk%2FDRM4Bd9hAQZYOdVgM%3D* OP-AMPhttps://eu.mouser.com/ProductDetail/Microchip-Technology/MCP6004T-E-SL?qs=JKWmWPM4biknesD%252BwnBNoA%3D%3D* VREG 3.3Vhttps://eu.mouser.com/ProductDetail/Microchip-Technology/MCP1754ST-3302E-CB?qs=sGAEpiMZZMsbRVlHDoeFZMrESykSd2vv9LUy8BDrECA%3D* VREG 5Vhttps://eu.mouser.com/ProductDetail/Microchip-Technology/MCP1754ST-5002E-CB?qs=sGAEpiMZZMsbRVlHDoeFZEcscB1e9H4uwwS8AaL1sLk%3D* VREF 3.3Vhttps://eu.mouser.com/ProductDetail/Microchip-Technology/MCP1501-33E-SN?qs=sGAEpiMZZMsyYdr3R27aV6Egp8EiTYjVLQN0yoD5dCA%3D* MCUhttps://eu.mouser.com/ProductDetail/Nordic-Semiconductor/nRF52840-Dongle?qs=gTYE2QTfZfTbdrOaMHWEZg%3D%3D

author: Kleinstein
author_rank: Super Contributor
timestamp: February 10, 2022, 05:41:40 pm
content: I would power the reference from the 5 V regulator a stable supply helps and is available already. The MCP1501 looks more like a low cost but also low performance one, so maybe at least plan for something better as an option (e.g. max6070, max6043 ) - that would be more suitable for the ADC.For a initial test board I would consider planing with dual OPs to have it easier to test a different OP type, maybe something faster / lower noise if needed.Unsoldering is much reasier with SO8 than SO14.Especially if not clear about the input circuit (AC coupled or DC coupled TIA) I would solder the parts locally in increments, e.g. only 1 channel to start, possibly already 2 different amplifier versions (AC / DC coupled).Edit: the resistor footprints are quite small and close together - not much fun to find suitable values under such condictions. Andsoldering is way easier with larger pads. Just in case one needs to go with a DC couple TIA one may want pads to have to AC coupling / level shifting at the TIA output side. For a test maybe 1 or 2 channels at least.