Many applications call for measuring ac lines or high dc voltages. One common technique uses a large voltage divider followed by a buffer. Another employs an inverting attenuator. The problems associated with both methods stem from uneven power dissipation in the resistors, poor system accuracy due to resistor mismatch, and a large noise gain.
A third solution yields high-accuracy, high-voltage measurements (Fig. 1). The integrator (OP177) supplies negative feedback around the difference amplifier (AD629), forcing its output to stay at 0 V. The voltage divider on the inverting input sets the common-mode voltage of the difference amplifier to VIN/20. VOUT, the integrator output and the measurement output, sources the required current to maintain the common-mode voltage. R1 and C1 compensate the system to a bandwidth of 300 kHz.
The transfer function is: VOUT = VIN/19. For example, a 400-V p-p input signal will produce a 21-V p-p output.
Figure 2 shows that the measured system nonlinearity is less than 20 ppm over the entire 400-V p-p input range. System noise is about 550 nV/√Hz referred to the input, or around 2-mV peak noise voltage (10 ppm of full scale) over a 300-kHz bandwidth.
