Relationship between aperture and sampling rate on PXIe-4145?

[scott_1111]

We are using a PXIe-4145 SMU to measure the average current over an aperture time of 2ms for our device. Does anyone know how the PXIe-4145 arrives at this average current value with regards to aperture and sample rate? Searching the NI site, I came across a document for the PXIe-4113 power supply that details the type of information that I need:

"The NI PXIe-4113 supports aperture times based on the power line frequency of the device. The NI PXIe-4113 acquires samples at a fixed rate of 1/10500 s, a single period of a 10.5 kHz clock. When you set an aperture time to acquire a measurement, the NI PXIe-4113 acquires the number of samples that occur during the aperture time, averages the samples, then returns one measurement that represents the average reading over the specified aperture time."

Unfortunately, I can't find equivalent documentation for the PXIe-4145. The specifications for the PXIe-4145 indicates a maximum sample rate of: 

(600 kS/s)/N where S is samples and N = 6, 7, 8, ..     <---(Not sure what N is here. Also samples?)

Also, in the PXIe-4145 specs, there a note that indicates: 

"When taking a measure record, only the Aperture Time property affects the sampling rate."

This statement seems to indicate that the sample rate is dependent on the aperture. Whereas, the excerpt from the PXIe-4113 document makes it appear that they are independent. Is there any document that clarifies all this for the PXIe-4145?

[santo_13]

You just need to configure aperture time as 2ms and the rest will be taken care of by the driver. User cannot determine or mess up the sampling rate of ADCs, it is just there as a piece of information.

To provide more information, not all SMU models have the same set of features. Having said that, in some simpler SMU/PPS the ADCs sample at a fixed rate and aperture time is achieved by averaging multiple samples. On advanced SMUs, the sampling rate of the ADCs is configurable as in the case of 4145, in this case, the aperture time will be a combination of averaging the samples and changing the sample clock based on the aperture time requested.

As for your question on what N means, it is just a set of possible values to derive the sampling rate from 600kS/s. This means, your best sampling rate is 100kS/s (since N starts at 6).

To clarify about the note, the measurement circuitry on the SMU is independent of the source circuitry, with that said, typically you link both together to measure automatically after every source complete event, in that case, though ADC can capture at 100kS/s, for each sample it will wait for the source unit to send a source complete event and hence, in reality, number of samples in 1s will not be 100k.

On the other hand, if you configure the measure when property to on-demand or trigger, the measurement circuitry is decoupled from source portion and hence can sample at the rate determined by aperture time.