Logging rate for multiple channels

[PhilG]

1. The documentation says, "The analog inputs has the following features, 500 kS/s aggregate input."   By aggregate, does it mean that if I record all 8 analog channels, the data rate is  (500 kS/s divided by eight channels = 6.25 kS/s per channel?

2. Suppose I record only one analog channel at the 500kS/s conversion rate. The documentation says the logger has a max logging rate of 400 kS/s. Thus the conversion rate is greater than the logging rate.  Will OpenLogger be able to log data for say 1000 samples without losing data or is the conversion rate effectively throttled down to the logging rate?

3. Digilent is calling their analog outputs "DC Power Supplies". Isn't the so-called power supply outputs powered by nothing more than  Digital to Analog Converter chips?  Why use the foreign terminology and just call them the industry typical DA outputs? You don't have to answer this one, just a peeve of mine.

[JColvin]

Hi @PhilG,

1. That is accurate; the aggregate input is divided based on the number of channels that are being read, so if you read all 8 channels, the sample rate ends up being 62.5 kS/s per channel (I think you missed a decimal point there). However, it is a bit more nuanced as the embedded ADC within the on-board MZ chip limits how channels are read. What I mean by this is if you choose to read only channels 3 and 7, the sample rate will be limited to 71.43 kS/s because the embedded ADC (by internal design that isn't changeable by a register setting) will read channels 1 through the last channel enabled (in this case channel 7) even if the in-between channels are not enabled. So if you want to read multiple channels, make sure they are on the lower analog channel numbers to minimize the impact to sample rate. A single channel enabled (even if it is channel 7) will still keep a 500 kS/s sample rate though.

2. It should have a max logging rate of 500 kS/s; I have corrected that bit of detail on the OpenLogger Resource Center, let me know if you see that elsewhere in the documentation. Originally, it was only going to be 400 kS/s but the firmware was streamlined to make 500 kS/s achievable.

3. Well, they aren't powered by Digital to Analog chips directly (it's done through through discrete components), which to be fair is all DAC's are made out of anyway. But I think the idea that the they have their own op-amps to produce the DC outputs (IC8 on page 6 of the schematic, https://reference.digilentinc.com/_media/reference/instrumentation/openlogger/openlogger_sch.pdf) makes it fair to call them power supplies. They are not beefy power supply lines, but do provide more current (50 mA) than a typical digital-to-analog converter (at least as far as I know).

Let me know if you have any questions about this.

Thanks,
JColvin