Circuit functions and advantages

The circuit shown in Figure 1 provides an 18-bit programmable voltage with an output range of −10 V to +10 V, an integral nonlinearity of ±0.5 LSB, a differential nonlinearity of ±0.5 LSB, and low noise characteristics.

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 circuit can provide high-precision and low-noise performance by combining precision devices such as the AD5781 , ADR445 , and AD8676 .

Reference buffering is critical to the design because the input impedance of the DAC reference input is highly code-dependent, resulting in linearity errors if the DAC reference source is not adequately buffered. The AD8676 has an open loop gain of up to 120 dB and has been verified and tested to meet the settling time, offset voltage, and low impedance drive capability requirements of this circuit application. The AD5781 has been characterized and factory calibrated so that its voltage reference input can be buffered using the AD8676 dual-channel op amp, further enhancing the reliability of the accompanying device.

This combination of devices delivers industry-leading 18-bit resolution, ±0.5 LSB integral nonlinearity (INL) and ±0.5 LSB differential nonlinearity (DNL), ensuring monotonicity, along with low power consumption, small PCB size, and high Cost-effectiveness and other characteristics.

Circuit description

The digital-to-analog converter (DAC) shown in Figure 1 is the AD5781, an 18-bit high-voltage converter with an SPI interface that provides ±0.5 LSB INL, ±0.5 LSB DNL, ​​and 7.5 nV/√Hz noise spectral density. In addition, the AD5781 has extremely low temperature drift (0.05 ppm/°C). The precision architecture used in the AD5781 requires forced sensing to buffer its voltage reference input to ensure specified linearity. The amplifiers chosen to buffer the reference input (B1 and B2) should have low noise, low temperature drift, and low input bias current. The AD8676 amplifier is recommended for this function, an ultra-precision, 36 V, 2.8 nV/√Hz dual op amp with low offset drift of 0.6 μV/°C and 2 nA input bias current. In addition, the AD5781 is characterized and factory calibrated to use the dual op amp to buffer its voltage reference input, further enhancing the reliability of the companion device.

In Figure 1, the AD5781 is configured in a gain of 2 mode, which allows a single reference voltage source to be used to generate a symmetrical bipolar output voltage range. This mode of operation uses an external op amp (A2) and on-chip resistors (see the AD5781 data sheet) to provide a gain of 2. These internal resistors are thermally matched to each other and to the DAC ladder, allowing ratiometric thermal tracking. The output buffer also uses the AD8676, 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 illustrated by the data in Figures 2 and 3, which show integral and differential nonlinearity as a function of DAC code. It is evident from the figure that these two characteristics are within the specifications of ±0.5 LSB and ±0.5 LSB respectively.

The total unregulated error of this circuit is composed of various DC errors, namely INL error, offset error, and gain error. Figure 4 shows a plot of total unadjusted error versus DAC code. The error is maximum when the DAC code is 0 and 262,143. This is expected and is caused by the absolute error in the reference output, the mismatch in the external resistors R2 and R3 (see Figure 1), and the mismatch in the AD5781 internal resistors RFB and R1 (see Figure 5).

The absolute error of the reference voltage is rated at 0.04%; the mismatch between resistors R2 and R3 in this example is rated at 0.02%; the mismatch between internal resistors R1 and RFB is rated at 0.01%. Therefore, the total gain error is 0.07% of full-scale range, or 184 LSB. Figure 4 shows a measured value of 20 LSB (i.e., 0.007% of full-scale range), indicating that all devices performed significantly better than their rated tolerances.

Noise 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. Figure 6 shows the peak-to-peak noise measured over a 0.1 Hz to 10 Hz bandwidth over 10 seconds. The peak-to-peak values ​​under the three conditions are 1.34 μV (mid-level output), 12.92 μV (full-scale output), and 15.02 μV (zero-level output). The mid-level output has the lowest noise, where the noise comes only from the DAC core. When mid-level codes are selected, the DAC attenuates the noise contribution of each reference voltage path.

However, a practical application 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 its passband; therefore, the measured peak-to-peak noise is more realistic, as shown in Figure 7 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. For the three conditions shown in Figure 7, the corresponding peak-to-peak values ​​are 1.61 μV (mid-level output), 43.33 μV (full-scale output), and 36.89 μV (zero-level output). The worst-case peak-to-peak value (43.33 μV) is roughly equivalent to ½ LSB.

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.