Measurement Computing PCM-DAS16x/16 - Manual (Page 14)

4 OPERATION
4.1 ANALOG RESOLUTION & RANGE
The 16 bit A/D converter provides a resolution of 1/65,536 parts of full scale. The
smallest reading of full scale (1 part in 65,536) is called a Least Significant Bit (LSB).
Accuracy is specified in LSBs. The PCM-DAS16x/16 is accurate to +/- 1.5 LSB.
Four different bipolar ranges are fully controlled by software. These are:
Bipolar 1LSB
+/-10V 0.000305V
+/-5V 0.000153V
+/-2.5V 0.000076V
+/-1.25V 0.000038V
The input range is controlled by a programmable amplifier and is completely under
software control.
4.2 CONVERSION SPEED & AMPLIFICATION
The A/D chip always runs at full speed. The A/D converter and sample & hold circuit
captures and digitizes a signal in 10 microseconds (10uS). The conversion speed of
the A/D remains constant in all conditions and at all throughput rates. This is impor-
tant. When you request a sample rate of say 20KHz, the A/D converter is still con-
verting the signal in 10uS. The 20KHz rate comes from the fact that conversions are
being initiated only every 50uS.
What factors limit conversion speed?
The first is clearly the A/D. A 10uS conversion speed translates to a maximum
throughput of 100KHz. The second limiting factor may be the analog front end.
The front end may consist of a multiplexor and a programmable gain amplifier. The
speed at which these circuits can switch may also limit the throughput of the A/D
board. That is, the rate at which it can acquire, convert and transfer a signal with full
accuracy. Accuracy is the key term here. The A/D can always run at full speed, but
has the front end settled and captured a true, accurate signal?
What about input range vs. speed?
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