How fast is the sweep channel speed of 34925A when using external reference to measure thermocouple?

Frequently Asked Questions (FAQs)

Summary

The sweep speed decrease when using enternal reference

Question

Answer

34925A is 420 channels/sec whether it is one card or eight cards，this drops the speed down to 315 channels/sec with the abus relays locked and 265 channels/sec without the lock when using external reference to measure thermocouple.

The maximum speed for thermocouple measurements with the 34925A is 420 channels/sec whether it is one card or eight cards. If necessary, there are some special commands to lock the analog bus and bank relays and make this go faster. The problem with this is that it increases the capacitance and thus reduces the settling time between channels. But this only affects large signal level changes between channels. If the channels are all thermocouple channels, those are all mV level signals, so settling won’t be a problem. Attached is a document describing how you lock the abus and bank channels. However, the lock only increases the speed to 471 channels/sec. The conversion from thermocouple voltage to temperature takes quite a bit of time. Also, this is for a fixed reference temperature which is faster than using an external reference temperature sensor that has to be converted from resistance to temperature. With an external reference sensor, this drops the speed down to 315 channels/sec with the abus relays locked and 265 channels/sec without the lock.

If you really need to go fast, just measure the thermocouple voltage and do the conversion to temperature in the computer. Use an external precision current source for the RTD or thermistor reference sensor, measure it’s voltage and convert it to the equivalent thermocouple voltage.

The speeds listed in the data sheet are for a single bank and they don’t list scanning speeds for temperature for anything except the 34921A.