Question

Question about adc, how would I find out how often my micro controller is sampling...is there an equation to determine the time difference(delta t) between samples. What would I need to know to determine this?

Answer 1

The sampling rate for an ADC is defined as the number of output samples available per unit time and is specified as samples per second (SPS). Two aspects of sample rate that must be considered when selecting an ADC for a particular application are the minimum sample rate and maximum sample rate.

Minimum sample rate: You may only look at the maximum supported sample rate of an ADC assuming that it will support all the lower sample rates. However, for some ADCs available today, this is not true. For example, most high-speed ADCs do not meet their specifications below a specified sample rate. This is due to the small capacitor used to hold charge at the input of ADC during conversion to support high sample rates. These ADCs have a higher leakage and cannot hold change for long time, thereby limiting the minimum sample rate for these ADCs. One way to address this is to ignore the samples in between the required samples. For example, if the required sample rate is 1ksps and the minimum sample rate supported is 2ksps, then every second sample can be ignored. The downside of this approach is that the dynamic power consumption of ADCs is proportional to the conversion rate. An ADC which is running at a lower speed may consume lesser power as compared to the ADC running at a higher clock that is ignoring samples.

Maximum sample rate: This parameter needs to be looked carefully when an ADC’s input channels are multiplexed. For ADCs like Flash converters and SAR, the sample rate for each channel can be calculated by dividing the specified sample rate by the number of channels. However, in ADCs like Delta Sigma where the input is first sampled at a very high rate and then decimated to provide the actual output, some samples need to be dropped to flush the decimator. This reduces the actual sample rate for each channel. If channels are being switched frequently then the actual sample rate can turn out to be very low for each channel.

Nyquist rate: As per the Nyquist sampling theorem, an analog signal should be sampled at least twice the maximum frequency content of that signal to achieve a faithful representation:

F_{s nyquist} = 2*F_signal

where, F_snyquist is the Nyquist sampling frequency and F_signal is the signal frequency.