This design features a voltage measurement signal chain with three measurement ranges: ±100mV, ±1V, and ±10V. As Figure 3-1 shows, OPA828 is the input amplifier and is in a non-inverting configuration for high-input impedance. A low-leakage multiplexer (TMUX6104) switches between three different gain settings.

Figure 3-1 TIDA-010970 Programmable Gain Amplifier Block Diagram

OPA828 is a JFET amplifier, offering a higher input impedance compared to bipolar input amplifiers, while having a lower 1/f noise than CMOS input amplifiers. Low 1/f noise is more important in dc measurements than broadband noise. The higher input impedance of the JFET amplifier is a practical trade-off even though bipolar amplifiers have lower 1/f noise than JFET. The high-input impedance prevents the measurement signal chain from disrupting the measured signal. The DMM connects to the load in parallel for voltage measurements. The load can experience a voltage drop if too much current from the circuit under test flows through the meter, resulting in an inaccurate measurement.

The gain on the input amplifier is programmable so the signal chain can accommodate three input ranges, 1V/V, 10V/V, and 99.8V/V, to scale the input signal to a 10V range. Each gain setting has a different bandwidth such that the minimum bandwidth is 15kHz at the largest gain. The different bandwidths across gains are not a concern since this design is intended to measure dc signals. Install capacitors C17, C18, and C19 to adjust the bandwidth as needed.

Each range must scale the input signal to a 5.5V signal or less because the ADS127L21B has a maximum recommended supply of 5.5V. This design scales the input signal to 4V because the ADC reference is 4.096V. The ADS127L21B power supply is ±2.75V. This power supply range is shared with THP210 and provides margin for the THP210 input common-mode limit.