ADC chip for multiple simultaneous conversions

Question

First off: I'm a mechanical engineer. I did do ME218 at Stanford, so I am quite conversant with electronics.

I know that there are basically three types of ADC architecture: sigma-delta, SAR and pipeline.

I have six analog signals that I need to convert to digital at the same instant of time. Since this is a mechanical device, the sampling rate can range from a few samples per second to a few kS/s - it is pretty irrelevant.

No, 'time stamping' is not quite working.

I might have nine more analog signals so (3 each of acceleration, gyro, mag, you get the reason for timing now?) This means fifteen analog signals, all to be converted at the same instant of time/on the same clock cycle.

What architecture of chip should I use?

Short of using an FPGA, is there any other way to do this using a single-chip ADC or microcontroller?

Answer 1

1. Use a simultaneous sampling ADC. Difficult hardware but easy software. Can get away with the lowest sampling rate.

2. Just sequentially sample through the channels with a multiplexed ADC. You can sample so much faster than a mechanical system's response that the error probably doesn't matter. Then just treat all samples taken in the same channel scanning cycle as simultaneous. Simplest hardware and software but needs the fastest sampling rate.

   For example, suppose we have 16-channels and a sampling rate of 1MSPS to spread across those channels. Let's assume our bandwidth of interest is 1kHz. To that end, let's pretend we are inputting the same 1kHz sine-wave (the highest frequency component in our bandwidth of interest) into all the channels. Between the two channels spaced farthest apart in the same scan cycle, the difference in the reading would differ by no more than 0.011% of full-scale (i.e. at the point where the sine-wave has the greatest slope). 0.011% of full scale exceeds 13-bit resolution.

   I was also being conservative choosing 1kHz. Although your mechanical bandwidth may exceed 1kHz, your sensors are probably just commercially available MEMs sensors for the smartphone or automotive industry and therefore do not exceed 300Hz.

   I seem to recall reading that it has been found from experience that military submarines require inertial measurements to employ gyroscopes and/or accelerometers with bandwidths in excess of 1kHz to acceptabley perform dead reckoning. That is a big huge submarine however, so the bandwidth to do the same on much smaller platform like what you are working on is probably higher. However, it's moot because the submarines requires very high grade optical gyroscopes to do this which your sensors certainly are not.

3. Sample channels sequentially like #2 and use zero stuffing and decimation to digitally interpolate data points as if you did simultaneously sample in hardware. Detailed process is here: https://www.ednasia.com/sample-multiple-channels-simultaneously-with-a-single-adc/ Simplest hardware but most complicated software. Can get away with a sampling rate between the other two methods.

Architecture is not important but if you need high speed sequential sampling or use an ADC integrated in an MCU (both of which will go hand in-hand if you are trying to simplify your hardware), you will probably end up with a SARs ADC. Don't forget your anti-aliasing filters.

Answer 2

As currently written, the question lacks a lot of important information, including the nature and bandwidth of the incoming signal.

That to one side, your maximum sample rate is the vaguely 'a few kS/s'. Let's take that to be 2,500 samples/sec max.

One low-cost solution is a single-chip ADC with 16 muxed inputs running at 1 Msps. That would be used to convert 15 channels in succession, giving 14 us of skew between the first and last conversion.

Since 2,500 sps gives you have 400 us between samples, 14 us of skew is 3.5% error by skew. At your low sample rates, this may well be plenty and acceptable.

Otherwise, you could use two ADCs with 8 muxed inputs at 1 Msps for 7 us skew or 1.75% error by skew. Or use a faster 16-channel ADC.

You can use an microcontroller (MCU) to control the ADC(s). You can also use an MCU with two internal ADCs, each with 8 muxed inputs. MCU ADCs tend to be lower resolution/quality than the dedicated ICs so you would have to assess the quality available against what you need.

Answer 3

The architecture of the ADC is not important here. What is important is that you have a sample-and-hold circuit for each channel and that you sample them all at the same instant in time. Once you have done that you can do the ADC conversions one at a time, with just one ADC, or in parallel with a separate ADC for each channel.

By the way, is ME218 supposed to mean something to us?

Answer 4

The answers given above cover the subject adequately, but I'd like to suggest another possibility. You can use any number of analog switches fed by the sensors through an appropriate op-amp, into high quality (low dissipation factor) capacitors, that are in turn monitored by multiple channel ADCs. At the moment you want to sample simultaneously, turn off all the analog switches, and then read each of them sequentially. Limitations will be due to the dV/dt of the sensor signal, resistance of the analog switch, size of the sampling capacitor, and the leakage current of the capacitor and connected components.