Circuit functions and advantages

The circuit shown in Figure 1 is a 20-bit linear, low-noise, precision unipolar (+10 V) voltage source that requires a minimal number of external components.  The AD5790 is a 20-bit, unbuffered voltage output DAC that operates from bipolar supplies up to 33 V. The positive reference voltage input range is 5 V to VDD – 2.5V, and the negative reference voltage input range is VSS + 2.5 V to 0V. Both reference voltage inputs are buffered on-chip and require no external buffering. The maximum relative accuracy is ±2 LSB, ensuring monotonic operation, and the maximum differential nonlinearity (DNL) is −1 LSB to +2 LSB.

The AD8675 precision op amp has low offset voltage (75 μV maximum) and low noise (2.8 nV/√Hz typical), making it an optimal output buffer for the AD5790. The AD5790 has two internally matched 6.8 kΩ feedforward and feedback resistors that can either be connected to the AD8675 op amp to provide a 10 V offset voltage, thus enabling a ±10 V output swing, or they can be connected in parallel to provide bias current cancellation. Function. This example demonstrates a unipolar +10 V output with resistors used for the bias current cancellation function. The internal resistor connection is controlled by setting the relevant bits in the AD5790 control register (see AD5790 data sheet).

The circuit's digital inputs are serial inputs and are compatible with standard SPI, QSPI, MICROWIRE® and DSP interface standards. For high-precision applications, this compact circuit can provide high-precision and low-noise performance by combining precision devices such as the AD5790 and AD8675.

Circuit description

The digital-to-analog converter (DAC) shown in Figure 1 is the AD5790, a 20-bit, high-voltage converter with an SPI interface that provides ±2 LSB INL, −1 to +2 LSB DNL, ​​and 8 nV/√Hz noise spectral density. In addition, the AD5790 has extremely high long-term linear error stability (0.1 LSB).

Figure 1 shows the unipolar buffer configuration of the AD5790. The output buffer uses the AD8675, which has low noise and low drift characteristics. This amplifier (A1) is also used to amplify the +5 V reference voltage provided by a low-noise precision voltage reference (in this case, a Krohn Hite Model 523 precision voltage reference). Resistors R2 and R3 in this gain circuit are precision metal sheet resistors with a tolerance of 0.01% and a temperature coefficient of 0.6 ppm/°C. For best performance over temperature, R2 and R3 should be in a single package, such as the Vishay 300144 or VSR144 series. R2 and R3 are both chosen to be 1 kΩ to keep system noise low. R1 and C1 form a low-pass filter with a cutoff frequency of approximately 10 Hz. This filter is used to attenuate reference noise.

Linearity measurement

The precision performance of the circuit shown in Figure 1 is demonstrated on the EVAL-AD5790SDZ evaluation board using an Agilent 3458A multimeter . Figure 2 shows that the integral nonlinearity is a function of DAC code and is within the specification of ±2 LSB (0°C to 105°C).

Figure 3 shows the differential nonlinearity as a function of DAC code within the specification range of −1 LSB to +2 LSB.

Noise drift measurement

To achieve high accuracy, the peak-to-peak noise at the output of the circuit must remain below 1 LSB, which is 9.5 μV for 20-bit resolution and a +10 V unipolar voltage range.

Real-time noise applications will not have a high-pass cutoff frequency at 0.1 Hz to attenuate 1/f noise, but will include frequencies down to DC in their passband; therefore, the measured peak-to-peak noise is shown in Figure 4. In this example, the noise at the output of the circuit was measured over 100 seconds, and the measurement fully covers frequencies as low as 0.01 Hz.

This measurement requires the use of a temperature-controlled, ultra-low noise voltage reference to prevent temperature drift from becoming a major factor in noise performance.

The zero-scale output voltage has the lowest noise, where the noise comes only from the DAC core. When zero-level code is selected, the DAC attenuates the noise contribution of each reference voltage path.

As the measurement time becomes longer, lower frequencies will be included and the peak-to-peak values ​​will become larger. At lower frequencies, temperature drift and thermocouple effects become sources of error. These effects can be minimized by selecting devices with smaller thermal coefficients.

To view the complete schematic and printed circuit board layout, see the CN-0257 Design Support Package: www.analog.com/CN0257-DesignSupport