Performance Characteristics of Digital Measurement Instruments

In this article, we discuss the performance characteristics of digital instruments i.e.

• Resolution
• Accuracy
• Linear errors
• Settling time
• Temperature sensitivity
• Monotonicity

Resolution

This is defined as the smallest increment in voltage that can be discerned. It is the reciprocal of the number of discrete steps in the Digital to Analog (D/A) converter input. Resolution depends on the number of bits i.e. the smallest increment in voltage is determined by the least significant bit (LSB).

Where n is the number of bits.

Accuracy

Accuracy is the measure of the difference between actual output and expected output. It is expressed as a percentage of the maximum output voltage.

Linear Error

Linearity means that equal increments in digital input of digital meters should result in equal increment in analog output voltage. In ideal case, where the values of resistances are very accurate, and other components are also ideal, we would have a perfect linear relation between output and input i.e. a straight line in an output-input graph but because of the fact that resistances used in the circuit have some tolerance, we can’t achieve a perfectly linear relation between input and output.

An example of a linear error is the offset error which is the output voltage when the digital input is 0000.

Settling Time

This is the time taken by the Digital to Analog converter to settle within ± ½ least significant bit (LSB) of its final value when a change in input digital signal occurs. The finite time taken to settle down to a new value is due to the transients and oscillations in the output voltage.

Fig settling time of a digital instrument

Temperature Sensitivity

The reference voltage supplied to resistors of a Digital to Analog (D/A) converter are all temperature sensitive hence, the analog output voltage depends, to some extend on the temperature. The temperature sensitivity of the offset voltage and the bias current of OP-AMP also affect the output voltage. The temperature sensitivity of a D/A converter is about ± 50 – ± 1.5 ppm/°C.

Monotonicity

A Digital to Analog converter is monotonic if it does not take any reverse step when it is sequenced over the entire range of input bits.