Circuit functions and advantages

The circuit shown in Figure 1 is an 18-bit linear, low-noise, precision bipolar (±10 V) voltage source that requires a minimal number of external components. The AD5780 is an 18-bit, unbuffered voltage output DAC operating 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 ±1 LSB, ensuring monotonic operation, and the maximum differential nonlinearity (DNL) is ±1 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 AD5780. The AD5780 has two internally matched feedforward and feedback resistors that are connected to the AD8675 op amp and provide a 10 V offset voltage. Therefore, with a single 10 V external reference, the output voltage can swing up to ±10 V.

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 AD5780, ADR445 , and AD8675.

This combination of devices delivers industry-leading 18-bit resolution, ±1 LSB integral nonlinearity (INL), and ±0.75 LSB differential nonlinearity (DNL) to ensure monotonicity, along with low power consumption, small PCB size, and high Cost-effective and other features, it is packaged in LFCSP.

Circuit description

The digital-to-analog converter (DAC) shown in Figure 1 is the AD5780, an 18-bit high-voltage converter with an SPI interface that provides ±1 LSB INL, ±0.75 LSB DNL, ​​and 7.5 nV/√Hz noise spectral density. In addition, the AD5780 also has extremely low temperature drift (0.005 LSB/°C).

In Figure 1, the AD5780 is configured in a gain of 2 mode, which allows a single voltage reference to be used to produce a symmetrical bipolar output voltage range. This mode of operation uses an external op amp (A2) and on-chip resistors (see AD5780 data sheet) to provide a gain of 2x. These internal resistors are thermally matched to each other and to the DAC ladder, allowing ratiometric thermal tracking. The output buffer also uses the AD8675, which has low noise and low drift characteristics. This amplifier (A1) is also used to amplify the low-noise ADR445's +5 V reference voltage to +10 V. R2 and R3 in this gain circuit are precision metal sheet resistors with tolerance and temperature coefficient resistors of 0.01% and 0.6 ppm/°C respectively. For best performance over temperature, R1 and R2 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-AD5780SDZ evaluation board using an Agilent 3458A multimeter . Figure 2 shows that the integral nonlinearity is a function of the DAC code and is within the ±1 LSB specification.

Figure 3 shows the differential nonlinearity as a function of DAC code within the specification range of −0.25 LSB to +0.75 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 76.29 μV for 18-bit resolution and a 20 V peak-to-peak 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 as low as DC in their passband; therefore, the measured peak-to-peak noise is more realistic, as shown in Figure 4 shown. 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. The upper cutoff frequency is approximately 14 Hz and is limited by the measurement setup.

Figure 4 shows the peak-to-peak values ​​for three conditions: 1.2 μV (zero-scale output), 32 μV (mid-scale output), and 64 μV (full-scale output).

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. In this circuit, the main source of low-frequency 1/f noise is the reference voltage source. In addition, the temperature coefficient value of the reference voltage source is also the largest in the circuit, which is 3 ppm/°C. To improve mid-scale and full-scale DAC output noise, a temperature-controlled ultralow noise voltage reference is required.

Figure 5 shows the signal chain performance after replacing the ADR445 with a +5 V Krohn Hite Model 523 precision voltage reference.

To view the complete schematic and printed circuit board layout, see the CN-0200 Design Support Package: www.analog.com/CN0200-DesignSupport