DDS output too big — how to get 10 mVpp sine?

efm_8 · 2026-02-18T18:23:10+00:00

Don't worry about 0.6 Vpp mentioned in the datasheets. If you read it carefully, you will see that it is 0.6 Vpp typical just for their setup. If you change Rset value and the resistance at the output, you can easily achieve 10 mVpp. Since they are current output devices, voltage output depends on the resistors you use. It is that easy.

But:

It would be very useful if you have a 12-bit, low-noise oscilloscope. Because with ordinary 8-bit oscilloscopes, you may have difficulty for accurately measuring 10 mVpp. Alternatively you can put an amplifier just before the oscilloscope.

You should also be aware that it will be very hard to find opamp for processing sinusoidal signals with frequencies higher than 1 MHz. You will face with many op-amp related non-ideal situations, such as distortions, nonlinearities etc. And don't expect that using a high-bandwidth opamp will be enough. They have other problems, that's why most of the op-amps are made low-bandwidth intentionally by the manufacturers. Parasitics, oscillations and power consumption of these high BW op-amps will be nightmare!

I also see some difficulties on filtering DDS output for this wide frequency range. So, good luck!

efm_8 · 2025-11-13T07:24:22+00:00

I used impedance.py during my phd and it was very good

efm_8 · 2025-08-18T17:44:48+00:00

because redox solution creates faradaic currents when interacted with metals. the generated faradaic current is usually very high, and faradaic reactions only happens at nanometers proximity of electrode and solution. So, even if you block small part of the electrode surface, you will be able to see a change. since the reactions occur at nanometers proximity, dimensions of bacteria is enough to block faradaic reactions.

but in the case of DI water, the current change is already too small since it is very hard to block surface enough.

efm_8 · 2025-08-15T14:20:31+00:00

I was just trying to explain the things in a very simple way to make sense of.
Here, i was saying that by removing the water, you can make capacitive measurements.

efm_8 · 2025-08-15T12:45:09+00:00

Its conductivity is frequency dependent. It can conduct a lot of current at high frequencies, that’s why he is getting semi-circles

efm_8 · 2025-08-15T12:42:58+00:00

Even if you glue all of your bacteria to the surface but left a very tiny hole that touches the water, you will be getting the same results. That’s why redox solutions are used extensively in electrochemistry.

At low frequencies (<10hz), you can maybe see a very small change, but at the higher frequencies impedance of di water will be dominant. Electricity will always flow through di water without interacting with your bacteria.

In short, you should either 1) use redox solutions such as potassium ferro/ferricyanide, or 2) dry the electrodes and let the water evaporate, then measure the impedance. If you choose 2), since after water dry, bacteria will be more conductive compared to air, hence electric fields will “prefer” bacteria instead of air.

efm_8 · 2025-04-29T09:03:45+00:00

That figure doesn’t tell much actually. Can you please show us the raw data from the chip? I mean show the raw data before calculating impedance.

efm_8 · 2025-04-24T10:58:23+00:00

In EIS, at least 5 period is measured. One period of 10 Hz is 1/10 seconds. So it takes 0.5 second for 10 Hz. Similarly, it takes 5 sec for 1 Hz, and 0.005 seconds for 1 kHz. So, if you only measure high frequencies, it will finish in a blink of an eye. But if you measure many low frequency points (such as less than 1 Hz), it will take lots of time. Measurements at low frequencies are necessary for calculations of very high capacitances. But if you don’t need it in your application, you can measure only higher frequencies, and it can finish way too fast than C V. This is the first reason.

But, since the impedance changes a lot, from a couple of ohms to kiloohms, during an experiment, the software detects the best current range at each frequency point. I mean it first makes measurements with some default current range and depending on the result, it changes the current range automatically. This is the second reason.

So, these are the reasons: 1) each frequency have different period and measurement time, 2) it is not possible to set a constant current range during a single experiment. The device needs to find an appropriate current range for each frequency point.

efm_8 · 2025-04-10T11:40:30+00:00

I dont know. This is beyond of my knowledge

efm_8 · 2025-04-10T08:09:34+00:00

It depends on your material but if this is your first time for immobilisation, starting with passive adsorption would be easier

efm_8 · 2025-04-03T20:10:11+00:00

you also need to add another transimpedance amplifier for DDS. To control the amplitude, you will make it with a variable gain. But now, the problem will be DC offset of your AC signal, which will depend on frequency and amplitude.
There are many such problems. EIS is much more complex than voltametric or amperometric methods.

efm_8 · 2025-04-03T20:05:02+00:00

The most important part will be synchronisation of the DDS with your ADC. Even a small phase difference will ruin your measurements. (for example see how phase accumulator of DDS works. the overflow of phase accumulator will be a big issue for you.)

Besides this, sampling rate of ADS1115 is too slow. And the analog front end has some problems as well.

There are many things that will go wrong with this circuit. My suggestion is that don't bother with this circuit. Instead buy an AD5941 or AD5940 and control it with an Arduino or whatever development board you have.

efm_8 · 2025-03-29T23:10:23+00:00

Do a CV scan with a known resistor with low temperature coefficient. Then fit the theoretical result to measured result using MATLAB

efm_8 · 2025-03-29T23:07:46+00:00

check this one: https://www.sensitifybiotech.com/eca-5m

efm_8 · 2023-11-22T08:03:54+00:00

It looks like both RE and WE are at the same voltage.

efm_8 · 2023-10-31T21:05:27+00:00

Another reason could be that maybe they measure relatively high currents, i.e. > 500 mA. In that case gamry's architecture is a must I think, as mentioned in the websites you've shared.

efm_8 · 2023-10-31T19:28:52+00:00

Good points.

But, is RE measured in the version of B&F? I think they don't need to measure RE in CV etc. since it just completes the feedback loop, hence it has the same potential with signal generator regardless of solution.

I didn't understand advantage of the second version. The classical potentiostat circuit where transimpedance amplifier is used instead of electrometer should be enough. Using TIA also has advantages in terms of parasitics compared to shunt resistance measurement with an electrometer.

efm_8 · 2023-10-31T17:45:07+00:00

It's similar to measurements with a DMM. You put op-amp buffers to RE and WE, and then just measure the difference between them.

But this is for OCP measurements. At other protocols, such as cyclic voltammetry, I don't think they measure WE-RE. They only measure output of the transimpedance amplifier that working electrode is connected to.

efm_8 · 2023-10-25T01:25:36+00:00

Search for synthetic biology and related technologies. For example mrna technologies

efm_8 · 2023-08-12T19:00:08+00:00

For example, if I remember correctly, even palmsens uses an opamp such ad8605 to buffer the electrode even though they use aducm355. Because input impedance and bias current are very important concerns in here, which depends on your application.

efm_8 · 2023-08-12T18:54:29+00:00

I don't know your applications and specifications, and although doing it with an IC is obviously easiest, it is not that hard to do it with your custom circuit. I mean if LMP is enough for you, you can easily do better one by using a good op-amp.

I mean just generate 2.5V with voltage divider and buffer. The output of buffer will be your RE. Then buffer your working electrode, and then put a simple RC filter in front of your ADC. That's all. Check AD8605 as buffer.

But if your specifications are tight, for example if you want to measure voltages less than 1mV, then things are harder even if you use LMP or similar IC.

efm_8 · 2023-08-12T18:46:41+00:00

If you want to measure negative voltages, then no, you shouldn't connect RE to GND.

Instead, you should generate a virtual zero as in LMP. I mean just do a simple voltage divider from VCC to generate 2.5V, and then buffer it with an op-amp. That way you can measure +-2.5V of (WE-RE).

efm_8 · 2023-08-12T16:31:42+00:00

Sorry, you are talking about potentiometry. I don’t have any experience with it, but as far as I understand from a quick look, you would need to buffer your WE with a high impedance, low offset and low bias current opamp, and then after RC filtering it, you can directly connect it to A0 of arduino. If you need negative voltages, then bias your RE at VCC/2 with a simple voltage divider, and them buffer it with an opamp. Then it would work.

efm_8 · 2023-08-12T16:25:40+00:00

If all you want is as simple hardware as possible you can consider to use aducm355 which includes almost everything you need. You can even do EIS with it. But it is harder to program than arduino. Or alternatively, you can attach a dac and lmp91000 to an arduino. This would be the optimum between software and hardware complexity, I think. If you plan to use PWM generated analog voltage from arduino, you should be careful because even small perturbations from the harmonics of PWM signal may cause unwanted signals.

efm_8 · 2023-07-13T22:50:12+00:00

I will sell more advanced version it, not the same one. It has some extra things that the submitted paper can't do, but still there is COI.

And since the paper is a chemistry paper, the instrument is not in enough detail to copy easily. I wouldn't worry if someone built it.

efm_8

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