Question from the customer:
"I want to know measurement speed of 7.5digits and 8.5digits like following data." (from the 3458A)
The answer to this question was difficult to obtain – The DMM is designed with two ADCs, which will provide the best performance over a broad range of applications. These are the Charge Balance Integrating ADC and the Successive Approximation ADC. So obtaining a chart like Keysight will be impossible.
The CB ADC is extremely linear – one of the best, of not the best linearity performance of all ADCs used in test instrumentation. A simplified circuit shows an integrating amplifier which measures both the input voltage being measured and a precision reference signal balanced against the integration of the measured signal.
The CB ADC is used for precision DC voltage and current measurements as well as ohms.
Aperture times are the time the input signal is measured during all the ADC measurement cycles making up an individual measurement. The range of aperture times can vary to balance needed measurement times as well as measurement quality (i.e. noise rejection effects), as noted in the statement.
To provide the best DC measurement performance a chopper amplifier is used to minimize measurement influence by the dmm on the parameter being measured – especially in the input impedance of the dmm and the bias current effects.
The Aperture time defines the time the unknown input signal is integrated by the ADC system (tu in the graph), and the initial ramp used in digitizing the captured signal (the initial downward slope of the td period.
The time of a single measurement sample is 110 microseconds (25 microseconds of balancing and 85 microseconds of integration of the reference ramps).
The sample can be repeated up to 4.7k times a second. The programmed aperture time sets amount of time (and number of 110 microsecond samples which are averaged) for a single measurement value.
This table shows how the number of bits in any ADC changes the resolution of the measurement range and the smallest measurement increment made by the ADC – or its sensitivity. This table is assuming the ADC is referenced to 10 volts.
For example at the 10 bit point there are 1024 different measurement points. The 10 to 12 bit range is what is used to typically measure 3.5 digit (1st line Top to bottom) measurements. A bench dmm of 6.5 digits (2nd Down) has about 20 or 21 bits of resolution, with one bit being representing about one millionth of full scale.
The 28 bit point is the ADC used in 8.5 digit in Red dmms. In this range the resolution is about 3 parts out of one billionth of full scale. This is the level of performance required for precision dmms used in metrology.