High speed DACs, such as ad9776 / 78 / 79 txdac series of analog devices, can provide differential output, but for low-end AC applications or high-precision level setting applications, the single ended current output DAC equipped with differential conversion circuit provides a novel method to generate differential waveform control function. The basic circuit in Figure 1 combines the current output DAC (i.e. IC1, such as 8-bit ad5424 DAC) and a single ended to differential operation amplification stages IC2, ic3a, iC3b – to produce the required output. For dual power applications, the single pole operation mode of the DAC can be selected to achieve the optimal performance of the DAC. The DAC uses a single operational amplifier to provide double quadrant multiplication or unipolar output voltage swing. The output of the DAC needs a buffer because changing the code applied to the input of the DAC will change its output impedance.

The following formula defines the output voltage of the circuit: Vout = – VREF × (D / 2n), where n defines the number of input bits, VREF is the reference voltage, and D is the decimal value of binary code. In order to generate a positive common mode voltage, the negative voltage can be used as the reference voltage of the DAC. The internal design of DAC can accommodate AC reference input signals of – 10 V to + 10 v. In this mode, when you rely on a 5V power supply to power the DAC, it provides a maximum update rate of 5msps for quarter full-scale code changes. Use resistors R1 and R2 only if your application requires adjustable gain.

The single ended to differential stage consists of two cross coupled operational amplifiers, and resistors R5 and R6 are configured as a unit gain follower. In order to realize the symmetrical circuit, each output is also driven by R7 and R8 as a unit gain inverter. The voltage applied to the positive terminal of the operational amplifier IC2 sets the common mode voltage of the circuit. Resistors R3 and R4 control the magnitude of the differential voltage. Please pay attention to the output load requirements of the application and the input voltage and output voltage capability of the operational amplifier.

For single supply applications, the current output DAC can be used in reverse mode, where the reference voltage Vin is applied to the iout1 pin of the DAC and the output voltage is obtained from the VREF terminal of the DAC (Fig. 2). In this configuration, the positive reference voltage generates a positive output voltage. The circuit does not use the feedback resistor RFB of the DAC, and the connection between it and iout1 prevents stray capacitance effect. The reference input of the DAC has an impedance that varies with the applied code, so a low impedance source is required.

Please note that each switch in the DAC ladder circuit no longer has the same source to drain driving voltage, which limits the input voltage to low voltage. As a result, the on resistance of each switch is different, and the linearity of DAC is reduced. In addition, this mode also limits the maximum update rate to 1.5msps. You can use several parts of the dual operational amplifier to buffer the input of the DAC and amplify the output voltage of the DAC (Figure 3). The intended application of the circuit determines your choice of supporting amplifier. For low-speed precision applications, Operational Amplifiers need very low input bias current and input offset voltage to avoid the deterioration of DNL (differential nonlinearity) performance of DAC. For example, the ad8628 provides a maximum bias current of 100Pa at room temperature and a maximum input offset voltage of 5mv. The low-frequency noise of operational amplifier is very important in the application of precision level setting, while the 0.1 Hz ~ 10Hz noise specified in ad8628 is lower than 0.5mv p-p. Its full swing input and output make it very suitable for single power supply circuits.

For high-speed system applications, the conversion rate of operational amplifier shall not dominate the conversion rate of DAC. The bandwidth of the operational amplifier must be wide enough to drive the feedback load, and the total bandwidth of the circuit must not be limited, and the output voltage stabilization time of the DAC should determine the maximum update rate of the circuit. The ad8042 in Figures 1 and 2 provides 170mhz bandwidth and 225v / MS conversion rate, making it easy to achieve these results. Other high-speed operational amplifiers, such as ad8022, ad8023 and ad8066, also work well in this application.

DAC only consumes 0.4ma power supply current, so the operational amplifier dominates the power consumption of the circuit. In order to minimize the footprint of the circuit on the printed circuit board, you can use a single ad8044 four core operational amplifier to replace all four operational amplifiers in Figure 2. At 1.4V common mode voltage and 0.6V differential signal, the single ended to differential conversion of digitized 8-point sine wave produces differential output (Fig. 3).

Responsible editor: GT

Leave a Reply

Your email address will not be published.