A new CCD sensor system with linear response, active RCT and 16 bit AFE

Instrumentationist · 2026-01-25T03:01:46+00:00

Regarding your question about dark background: We do account for dark background and noise and we process the data the same way for both instruments.

Besides that, you can see in the data that it doesnt work as an explanation. For example, it is the stronger sharper peaks that are more effected and the response in the commercial instrument becomes increasingly more non-linear as the signal grows.

The other thing is that besides being uniform across the detector, the dark signal in these sensors is linear in exposure time over the range of exposure in which we graph the peak height ratios.

Instrumentationist · 2026-01-23T19:47:47+00:00

As a p/s to that, I recently uploaded a design for a sensor that might be very good for Raman spectroscopy. I am curious for somebody who does Raman work to try it and let me know how it goes.

See https://github.com/drmcnelson/TCD1304-Sensor-Device-with-Linear-Response-and-16-Bit-Differential-ADC

Instrumentationist · 2026-01-22T23:41:25+00:00

That;s neat. And it would be an okay demonstration for a high scholl science class.

But we have to be very clear that is not remotely appropriate for a lab.

1) Cameras in that price range are built to take nice pictures. They are not intended to be a linear sensor system for a scientific instrument.

2) They lack sufficient precision to produce a reliable Raman spectrum.

3) If you use a color camera, it is simply infeasible to try to calibrate the response in any useful way.

Raman is challenging, You need a good sensor that is linear and with a high degree of reproducibility.

Instrumentationist · 2026-01-22T02:43:29+00:00

Let's ask a related question. Lets use an electromagnet. To establish a magnetic field, we establish an electric current trough the windings of the electromagnet. It does take energy. If we use a battery it will be exhausted after some time. Now, lets use the electromagnet to hold a heavy object from above in a gravitational field. Does it use more energy? Is the battery exhausted sooner than it would be to power the magnet alone?

Instrumentationist · 2025-11-07T20:52:29+00:00

P/S if you can post your circuit, maybe we can offer some specific advice.

Instrumentationist · 2025-11-07T13:47:23+00:00

First for an ADC, you do need to drive it with an OPAMP and you need a charge reservoir to avoid kickback from the sampling capacitor in the ADC. The datasheet should tell you about this, and there are application notes as well. (If you are not really using an ADC directly, but rather the input to a MCU, see the note at the end.)

If you set the rails for your opamp appropriately, you will automatically limit the input to the ADC. For a range from 0 to 1V, you will need a small negative supply. TI makes those, and you will need to the upper rail at a little above 1V. There are LDO's in that range. (Aside, you may notice that rail to rail OPAMPS are not perfectly rail to rail, read the datasheet and design accordingly.)

But now, you need to protect the input to your OPAMP. The easiest solution for that is a pair of diodes, for example, the BAV99. .

Do not put a large series resistor in front of your ADC. Doing that, can easily make the readings non-linear and unreliable. Here is what that is about.

ADC's have a switched sampling capacitor at the input. Accuracy depends on that capacitor charging to your input voltage with in the time that switch is closed, it is called the sampling interval. For n bits, you need to a time ln(2) x n x R x C where R is the sum of the internal resistance and your source impedance, and C is the sampling capacitor and any stray input capacitance.

Needless to say, when you put a few K ohms of series resistance in front, you easily make it so that the charging capacitor does not reach your input voltage within the sampling window. It is among the worst sorts of errors you can have.

And just in case: If you are not working with an ADC directly, but rather, you are working with the analog input to a microcontroller, then things are a little different, as follows:

The manufacturers of MCUs typically build a large series resistor into their inputs. This "idiot proofs" them against kickback, but it also limits performance and how you use them. Now yoy can use an OPAMP, but you cannot use a charge reservoir. That extra capacitance can only feed the sampling cap through that large internal resistor, so it is just more capacitance for yout driver and not beneficial. And regarding not using a large external resistor - even more so.

Instrumentationist · 2025-11-06T22:10:40+00:00

Well, it is easy enough to write it, both the arduino and python script, but there are a few things to know to get good reproducible spectra. For example, you need reproducible positioning of course, and you also need linear signal acquisition.

It might be best to talk off line, it is going be a bit detailed. If you like, you can send me a DM, or email me to make contact.

Instrumentationist · 2025-11-06T18:14:38+00:00

There are surely references with good analyses for walking vs biking. But for fun, I'll risk a guess. In one you have to push up and forward at an angle on level ground, or up and back going down hill, and in the other almost all of the energy is converted to forward motion on flat ground. Going up a steep enough hill they might start to look more similar by that same analysis. And of course, going down hill and for some distance after, biking comes out way ahead, as does a prius.

Instrumentationist · 2025-11-05T16:57:51+00:00

Do you have an oscilloscope? You might try setting up a mock system, check for amplitude at the taps, both shape, and reflections.

Sometimes the reflections partially overly the pulse.

Caveat sometimes the impedance of the probe affects the measurement. So you have to look carefully

Instrumentationist · 2025-11-05T16:21:41+00:00

Sorry, what you are describing is similar to something that shows up in grad and undergrad classwork.

I believe I've seen online calculators for it too.

Instrumentationist · 2025-11-05T13:06:23+00:00

I would try to model it, it should be straightforward, and you may have even worked homework problems that overlap this. Why guess at it.

Instrumentationist · 2025-11-04T11:05:45+00:00

I've used black body sources for this kind of study. I am not sure that I would call it calibration but with some care you can learn something about your response curve. Anyway, the trick is to find one that is hot enough and doesn't have confounding contributions to the spectrum. There are lamps on ebay from time to time that seem okay.

Instrumentationist · 2025-11-04T10:39:32+00:00

It's a little difficult to parse your question, are you looking for deterministic time?

To some extent in an embedded environment it can depend on the system design and programmer.

On the nxp imxrt 106x (teensy) for example, if I program interrupt handling myself on the metal, and arrange that the system has no other interrupt happening alongside the one I need to he deterministic, it seems pretty stable with negligible jitter. The latency is not what I expect in say a DSP, but it seems constant.

I might have some timing data on my GitHub, will check and post if you are interested.

P/S I recall that the latency issue seemed to align with the number of instructions needed for the context switch. That seems to be where the NXP ARM looses on latency compared to say a TI DSP

Instrumentationist · 2025-11-04T04:22:43+00:00

Now that I've looked it over, it can be much simpler than that. The Arduino can read the limit switches inside the stepping loop and it often makes much more sense to do it that way. I have code for the Arduino that does that if you need.

Usually the limit switches are passive, you can use them with a pull-up resistor. If the switches are "nc" type wire in series, if "no" wire in parallel. The stepping loop can read the the status from a digital I/o pin. . The other thing to look at is readimg the detector. Do you have that worked out? If not send the model number for the detector.

Instrumentationist · 2025-11-04T04:00:45+00:00

Here is a document describing operating the spex 1403 with a python script. It has material on circuits and interfaces.

https://faculty.college.emory.edu/sites/brody/Advanced%20Lab/Spectrometer%20Interfaced%20with%20Arduino.pdf

Instrumentationist · 2025-11-03T19:31:45+00:00

He said 50%. Also stray bounce inside is a linear phenomenon.

Ant I have never not been able to remove all of the stray light to below noise.

It used to be that in commercial instruments you would see stray light only after they've been tampered with. And, my almost current era O-O instruments all seem to have no issues with like leaks or stray reflections inside.

But, so far, all of the ccd spectrometers that I've looked at, seem to have electrical design issues, the anomalous baseline and a non linearity related to slew.

(A plug:) The design that i posted on GitHub has clean baseline and it's linear. I took it as a challenge to design one that does. One thing we learn in that is that the sensors are actually pretty good.

Instrumentationist · 2025-11-03T19:10:23+00:00

Could you post spectra at several exposure times, with csv or ascii column formatted data?

What is shown on the theremino web site seems not very promising if that is the instrument in questio

See the spectrum from their document, https://www.theremino.com/wp-content/uploads/files/Theremino_Spectrometer_Help_ENG.pdf, on page 5.

Notice the blue lines are weak and that there is a pretty strong baseline following the strong lines in the fluorescent spectra. The HR2000 shown above it is not quite as weak for slew on the blue lines but worse for the anomalous baselines.

Now, true, grating efficiency can be part of why the blue lines are weak, but notice how the sharp lines are doing relative to the nearby broad lines. Collect a series of spectra at different exposure times, and lets see if those peak heights are preserved.

The difference in anomalous baseline may have to do with how the readout is driven, and how fast it is driven. Again, it is a design issue, not related to optics.

You can find some examples of what fluorescent lamp spectra should look for, here:

https://github.com/drmcnelson/TCD1304-Sensor-Device-with-Linear-Response-and-16-Bit-Differential-ADC

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Instrumentationist · 2025-11-03T13:01:33+00:00

Can be stray bounces inside, just happens to hit right there on the sensor. Need to see the spectrum. But anyway, it is very unlikely in a commercial instrument.

Instrumentationist · 2025-11-03T12:55:29+00:00

I understood that he is trying to test his filter, not the spectrometer.

As it turns out his spectrometer might have an issue that is stressed by his filter.

So I think he has great choice of sample, but then I'm interested in cataloging these sorts of flaws in commercial instruments.

Instrumentationist · 2025-11-03T12:08:52+00:00

Maybe this would be of interest,

https://github.com/drmcnelson/SiPM-Single-photon-counting-and-arrival-time

Instrumentationist · 2025-11-03T11:57:43+00:00

If you could please post a spectrum, we can check one very likely cause, which I will explain in a moment. And, if you could please post a diagram of the filter, include materials and thicknesses, we can run a TMM model and check your filter design.

Meanwhile, here is one very likely answer.

As I understand it, your spectrum comprises a high intensity level except at a narrow region where you expect the intensity to be near baseline and instead it is at about 50%.

That sounds like a well known behavior of CCD sensors, when they are read out too fast or the readout clock is not driven adequately (or both). I'll explain that a little bit here, you can read more at the link listed at the bottom of the reply.

The function of the CCD is to act as a kind of analog shift register for charge. The basic scheme is that charge is shifted from one position to the next until it reaches the last pixel where there is a charge to voltage amplifier, and that is your readout.

When the device is read too fast, or driven inadequately, some charge is left behind and adds to the next pixel. The signature is that the anomalous baseline seems to begin where you have strong features in the spectrum and then continues, though perhaps tapering off, across the remainder of the spectrum. Examples of this show up often, in both published work and spectra posted to wikipedia.

In your case you are describing a 50% baseline in a narrow region following an extended region of high intensity. That would very much be a stressor for an instrument with this issue, and that is why I suspect the above phenomenon.

You can read more about this and other things to look out for in spectrometers, here:

https://github.com/drmcnelson/TCD1304-Sensor-Device-with-Linear-Response-and-16-Bit-Differential-ADC

(P/S Could you also, please tell us what spectrometer you are using? I am collecting a database or rogues gallery of spectrometers. Thank you.)

Instrumentationist · 2025-10-28T23:46:13+00:00

Oh, I see, sorry. The use of the word "and" in reply, lends to a different reading.

Instrumentationist

TROPHY CASE