Abstract
Methods are described for measuring the settling times and other dynamic characteristics of voltage and current output D/A converters (DAC's), operational amplifiers, and precision voltage step generators. Circuits are described for measuring voltage-output device settling times as short as 1 μs to within a settling error of ±2 ppm, and current-output device settling times as short as 40 ns to within a settling error of ±0.012 percent.
I. Introduction
Settling time is an important measure of the dynamic performance of D/A converters (DAC's), operational amplifiers, and precision voltage step generators. Settling time for these devices can be defined as follows (see Fig. 1): Following a prescribed input change, usually full-scale range (FSR), the settling time of the device is the time required for the output to reach and remain within a given error band whose center coincides with the final value. Settling error is ±½ the error band. For DAC's, the error band is usually ±½ least significant bit (LSB). Note that part of the settling time is time interval t − t, the delay of the output relative to the input. This definition is also applicable to voltage step generators, triggered at time t.
An additional measure of the dynamic performance of these devices is the offsets, thermal transients, and other recovery time effects caused by changes in repetition (or switching) rate, duty cycle, and output amplitude. These effects, as well as settling time, cause the device accuracy to be different under severe dynamic conditions than under, say, quasi-static conditions. Therefore, it is useful to have test methods for making accurate relative measurements as the device is exercised over a range of operating conditions.
This paper describes two circuits for measuring the settling time of voltage-output devices to within a settling error of ±2 ppm of FSR, and a circuit for measuring the settling time of fast current-output devices to within a settling error of ±0.012 percent of FSR. These circuits are generally suitable for comparing a device's voltage or current output immediately after a transition with the output at times up to at least a second after the transition, where essentially static conditions prevail.
II. Measurement methods
A. Choice of oscilloscope
The most detailed examination of voltage steps can be made using a wide-band oscilloscope with a sensitive vertical amplifier. Oscilloscopes designed for "real-time" measurements are available with maximum vertical deflection sensitivities ranging from 10 μV/cm to 2 mV/cm. Corresponding bandwidths are typically 1 and 200 MHz. Deflection sensitivies as high as 20 or 50 μV/cm are useful for settling time measurements. Since the voltage step illustrated in Fig. 1 can be as large as 20 V, devices for measuring settling time often employ a biased voltage divider to shift level V to near zero potential and use various circuits to limit the output signal prior to the voltage transition. This shifting and limiting permits accurate measurement of voltage variations near the V level and avoids overdrive of the oscilloscope prior to the transition , . However, the effect of the transient that results during the transition V to V must be carefully evaluated.
Sampling oscilloscopes, which display the input signal in "equivalent time," have a maximum vertical sensitivity of approximately 2 mV/cm and a bandwidth of at least 1 GHz . Since the minimum sampling period is sufficient to allow an overdriven vertical amplifier to recover between samples, the problem of overdrive is minimal for sampling oscilloscopes. Therefore, very fast voltage steps as large as (approximately) 2 V can be examined without limiting . To prevent loading of the signal source by the low input impedance (50 Ω) of these units, an interfacing field-effect transistor (FET) probe or specially designed amplifier with high input impedance and 50-Ω output impedance is usually used. However, either a probe or amplifier may be susceptible to overdrive and adds noise and instability to the measurement system. Also, since sampling oscilloscopes are not suitable for observing low-frequency data, they cannot be used to compare the dynamic performance of a device with its quasi-static performance.