Composite Amplifier Boosts Precision Of Programmable Voltage Source

July 8, 2002
Engineers often need dc voltage sources with very high precision and good resistive and/or capacitive load-driving capability for industrial applications. However, precision benchtop power supplies can be very expensive if high accuracy also is...

Engineers often need dc voltage sources with very high precision and good resistive and/or capacitive load-driving capability for industrial applications. However, precision benchtop power supplies can be very expensive if high accuracy also is required. IC-based dc voltage sources, such as references and regulators, cost less but only partially satisfy the requirements. References have good dc accuracy yet lack resistive or capacitive load-driving capability. On the other hand, regulators have the load-driving power but not the necessary accuracy and software programmability.

A power output stage that can drive capacitive loads is shown in the figure. It has a precision input circuit and voltage programmability from ­2.5 to +2.5 V, with 16-bit accuracy. The circuit employs a dual auto-zero amplifier, A1a-A1b (AD8552); a high-output current amplifier, A2 (AD8531); a 16-bit serial-input digital-to-analog converter (DAC), A3 (AD5542); and a precision voltage reference, A4 (AD780). If programmability isn't needed, the circuit can use a fixed reference instead of the DAC.

Circuit operation goes as follows: The DAC (A3) is configured for bipolar operation when combined with A1a. Its output can be programmed to ±VREF, which is set to 2.5 V by the AD780. In combination, the DAC and amplifier produce a negligible error. Worst-case, bipolar output-voltage error, VOUT-BIP, can be calculated as shown in the Equation Listing, where:

D = code loaded to the DAC
VREF = reference voltage = +2.5 V
VGE = gain error = ±5 LSB
ZE = zero-scale error = ±1 LSB
INL = integral linearity = ±1 LSB
VOS = amplifier offset voltage = 5 µV
RD = resistor matching = ±0.00076%
A = amplifier open-loop gain
   = 1,000,000

For example, when the code loaded to the DAC is 1 LSB (code 1000 0000 0000 0001), the worst-case error will be approximately 350 µV. These small errors can be stored as CAL factors in a calibration table if even higher accuracy is required.

The programmed voltage is fed into the unity-gain composite amplifier composed of A1b and A2. This composite amplifier performs close to the ideal, as the input stage of the AD8552 (A1b) provides a very high common-mode rejection ratio (CMRR), power-supply rejection ratio (PSRR), open-loop gain, and very low offset voltage.

The composite amplifier also has a very high output current and good capacitive and resistive load-driving capability due to the AD8531. Additionally, both the AD8552 and AD8531 have rail-to-rail inputs and outputs. Using the equation below, users can calculate the worst-case error of this composite amplifier:

VOUT = (VIN ­ VIN_DIFF)
VIN = VOUT_BIP
VIN_DIFF = VOS + en + VOUT/A +VICM/CMRR +ΔVS/PSRR

where:

en = amplifier input noise = 20 nV/√Hz
CMRR = 120 dB
PSRR = 120 dB

When light loads are driven (RL > 500 Ω), the composite amplifier circuit adds less than 10 µV of error to the programmed signal coming out of the DAC. For larger than ±120-mA output currents, A2 limits the output performance.

The above circuit can function as a precision, programmable reference with sink and source output current, or as a low-dropout regulator with excellent load regulation and efficiency. Proper bypassing of the am-plifiers and additional pins on the DAC and DSP are omitted for clarity.

See associated figure.

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