The reading rate of a DMM is derived from the measurement cycle (Reading Rate = 1/Measurement Period). All of the phases in the measurement cycle are set by the driver to an optimized setting depending on the measurement being used. Reducing or removing any part of the measurement cycle will increase the reading rate. However doing this may have an effect on the resolution and accuracy of the measurement, but with careful choices you can minimize the affect of these changes.
There is usually many different ways to achieve a desired reading rate. Settling Time and Aperture Time are usually the two longest phases in the measurement cycle so if a large change in speed is desired consider modifying these phases. Below all of the measurement cycle phases are described along with some guidelines on when to modify them.
The Switch Time is set by the driver and cannot be programmatically changed. It is usually a very small value.
ADC Calibration (407x only)
By default ADC Calibration is ON when taking measurements at 6.5 and 7.5 digits of precision. If absolute accuracy is not as important for a particular test and only the change in the signal is desired, then this could be turned off to increase measurement speed.
Auto Zero (adjustable on 407x only, always enabled on 4065)
Similar to ADC Calibration, Auto Zero is by default turned ON when using 6.5 and 7.5 digits of precision. Auto Zero may be turned off to increase measurement speed but this may lower the absolute accuracy. With the 407x, Auto Zero can also be set to ONCE to only perform Auto Zero one time at the beginning of a multiple point measurement to help increase speed.
Default settling times vary based on the function and range specified. The longer the analog to digital converter has to settle, the more accurate a reading it will produce. The settling time is more important when measuring larger signals that vary in time or when measuring high resistances. If measuring small signals that are fairly consistent then lowering the settling time can be a good way to increase measurement speed.
The larger the aperture time, the better the resolution. Select short aperture times for faster measurement speed.
During the aperture time many measurements are taken by the DMM and basically averaged together to return a single sample. Acquiring more points and averaging them usually cancels out more noise. If a system is in a low noise environment and proper shielding and cabling is used then the aperture time may be lowered without experiencing a large difference in resolution. Another source of noise is powerline noise and the aperture time can be set to a multiple of the powerline to cancel out any 50 or 60 Hz noise. (1 powerline cycle (PLC) at 60 Hz is 16.67ms)
By manually setting the aperture time the absolute resolution value is ignored. Therefore if you specify a 6.5 digit measurement earlier in your code this programmatically determines the aperture time. If you then manually set a smaller aperture time then you may no longer be at 6.5 digits since the aperture time is related to the resolution. However, even though the two are related, the aperture time can still be tweaked to achieve faster reading rates while maintaining the same resolution depending on the measurement.
By adjusting the above measurement cycle phases a DMM can be optimized to not only acquire at a specified resolution but also at a faster speed. Many times a small change is needed to one or more of the phases in order to achieve the maximum speed stated in the specifications.