CN0531

Digital to Analog Conversation

The core of the circuit shown in Figure 1 is the AD5791, a 20-bit digital-to-analog converter (DAC) with a serial peripheral interface (SPI), offering ±1 LSB INL and ±1 LSB DNL performance and 7.5 nV/√Hz noise spectral density. The AD5791 also exhibits an extremely low temperature drift of 0.05 ppm/°C. The precision architecture of the AD5791 requires force sense buffering of its voltage reference inputs to ensure specified linearity.

The amplifiers chosen to buffer the reference inputs must have low noise, low temperature drift, and low input bias currents. The recommended amplifier for this function is the AD8676, an ultraprecision, 36 V, 2.8 nV/√Hz, dual op amp, exhibiting a low offset drift of 0.6 µV/°C and an input bias current of 2 nA. In addition, the AD5791 is characterized and factory calibrated using this dual op amp to buffer its voltage reference inputs, further enhancing confidence in this combination of components.

The circuit can be software configurated in two modes, bipolar or unipolar. Figure 1 shows the bipolar configuration in which the output voltage ranges between −5 V to +5 V by using a single 5 V reference buffered by the AD8676. To determine the output voltage in bipolar mode, use the following equation:

where:
V_OUT is the output voltage.
V_REF is the reference voltage (5 V on CN-0531).
Code is the 20-bit code programmed into the DAC.

When configured in unipolar mode, R1 is internally connected in parallel with R_FB, and the output range is 0 V to 5 V (see Figure 1). To determine the output voltage in unipolar mode, use the following equation:

Voltage Reference

A programmable voltage source is only as good as its voltage reference, which determines the output span of the source. The extremely small 0.05 ppm/°C gain drift of the AD5791 results in the voltage reference becoming a limiting factor for gain accuracy and drift in most applications.

The LTC6655BHLS8-5 provides a highly stable and accurate 5.0 V reference for the CN0531. This reference has an initial accuracy of 0.025% with 2 ppm/°C maximum temperature drift, and 0.25 ppm noise (0.1 Hz to 10 Hz). The hermetic package is insensitive to humidity and provides an outstanding thermal hysteresis of 30 ppm.

Long-term drift is exceptional, typically 20 ppm/√kHr. Long-term drift cannot be extrapolated from accelerated high temperature testing, which is an erroneous technique that delivers drift numbers that are wildly optimistic. The only way long-term drift is determined is to measure the drift over the time interval of interest, and the LTC6555 has undergone extensive characterization over thousands of hours of operating time. Refer to the LTC6655 data sheet and Design Note 229 for further information on these methods.

The level of performance of the LTC6655 is rivaled only by more exotic and expensive voltage references that employ heaters, closed-loop temperature control, and insulated packaging, thus this device is an optimal combination of performance, power, and cost for the CN0531.

Output Buffer Stage

The AD5791 employs a voltage output, segmented R-2R DAC architecture, which inherently has excellent full-scale and zero-scale error, and the INL and DNL both have a 1 LSB maximum. The output is unbuffered with a constant resistance of 3.4 kΩ, requiring additional signal conditioning in most applications, ideally preserving the dc accuracy and noise specifications of the AD5791.

The AD8675 is a suitable amplifier with an 10 μV typical offset, a 0.6 μV/°C maximum drift, and a 2 nA maximum bias current.

By default, the AD5791 RFB path is connected to the output of the buffer. For applications requiring higher drive capability or remote sense, the on-board connection can be opened and feedback connected remotely at the output of a power buffer stage, for example.

Power Tree

The CN0531 power tree, shown in Figure 2, consists of an LT3471 boost/inverting converter followed by the LT3042 (positive) and the LT3093 (negative) ultralow noise, ultrahigh PSRR linear regulators.

The LT3471 allows the circuit to operate from a single 3.3 V or 5 V supply provided by the development platform board. The output voltages are set to ±8 V, which are then regulated to ±7.5 V by the LT3042 and the LT3093. The LT3042 and the LT3093 provide an exceptional 80 dB and 75 dB PSRR, respectively, at the 1.2 MHz switching frequency of the LT3471, ensuring minimal switching feedthrough at the DAC output.

CN0531 Linearity Measurements

The following data shown in Figure 3 demonstrates the precision performance of the circuit shown in Figure 1 configured in bipolar mode by using a 6½ digit, Truevolt digital multimeter (DMM). Figure 3 shows the INL of the CN0531 as a function of the DAC code. The results are within the AD5791 data sheet specifications of ±1 LSB.

Noise Calculations and Measurements

The 7.5 nV/√Hz output noise spectral density of the AD5791 is entirely due to its 3.4 kΩ source impedance. The AD8676 output buffer has a voltage noise density of 2.8 nV/√Hz and a current noise density of 0.3 pA/√Hz, producing a negligible 1 nV/√Hz looking into the output resistance of the AD5791. The total noise of the DAC and output amplifier is then

This value is the expected noise when the output is set to 0 V. As the DAC output increases, the noise is dominated by the LTC6655BHLS8-5 reference. Figure 4 shows the output power spectral density for an output voltage of 5.0 V.