CN0028 
Precision, Bipolar, Configuration for the AD5547/AD5557 DAC

Figure 1: 4Quadrant Multiplying Mode, VOUT = –VREF to +VREF
This circuit uses the ADR01, which is a highly accuracy, high stability, 10 V precision voltage reference. As voltage reference temperature coefficient and longterm drift are primary considerations for applications requiring high precision conversion, this device is an ideal candidate.
An op amp is used in the currenttovoltage (IV) stage of this circuit. An op amp’s bias current and offset voltage are both important selection criteria for use with precision current output DACs. Therefore, this circuit employs the AD8512 op amp, which has ultralow offset voltage (80 μV typical for Bgrade device) and bias current (25 pA typical). C9 is a compensation capacitor. The value of C9 for this application is 2.2 pF, which is optimized to compensate for the external output capacitance of the DAC.
The input offset voltage of the op amp is multiplied by the variable noise gain (due to the codedependent output resistance of the DAC) of the circuit. A change in this noise gain between two adjacent digital codes produces a step change in the output voltage due to the amplifier’s input offset voltage. This output voltage change is superimposed on the desired change in output between the two codes and gives rise to a differential linearity error, which, if large enough, could cause the DAC to be nonmonotonic. In general, the input offset voltage should be a fraction of an LSB to ensure monotonic behavior when stepping through codes. For the ADR01 and the AD5547, the LSB size is
The input bias current of an op amp also generates an offset at the voltage output as a result of the bias current flowing through the feedback resistor, RFB. In the case of the AD8512, the input bias current is only 25 pA typical, which flowing through the RFB resistor (10 kΩ typical) produces an error of only 0.25 μV.
The AD5547/AD5557 DAC architecture uses a currentsteering R2R ladder design that requires an external reference and opamp to convert to an output voltage. VOUT can be calculated for the AD5547 using the equation
where D = 0 to 65535 for 16bit DAC (D is the decimal equivalent of the input code). VOUT can be calculated for the AD5557 using the equation
where D = 0 to 16383 for 14bit DAC (D is the decimal equivalent of the input code).
These circuits can also be used as a variable gain element by utilizing the multiplying bandwidth nature of the R2R structure of the AD5547/AD5557 DAC. In this configuration, remove the external precision reference and apply the signal to be multiplied to the reference input pins of the DAC.
Samples
Product 
Description 
Available Product

ADR441  Ultralow Noise, LDO XFET® 2.5V Voltage Reference w/Current Sink and Source 
ADR441ARMZ ADR441ARZ 
AD5547  Dual Current Output, Parallel Input, 16Bit Multiplying DACs with 4Quadrant Resistors 
AD5547BRUZ AD5547CRUZ 
AD8605  Precision, Low Noise, CMOS, RRIO Op Amp (single) 
AD8605ARTZREEL7 
AD5557  Dual Current Output, Parallel Input, 14Bit Multiplying DACs with 4Quadrant Resistors 
AD5557CRUZ 
AD8512  Low Noise, Low Input Bias Current, Wide Bandwidth Precision JFET Dual Op Amp 
AD8512ARZ 
ADR445  Ultralow Noise, LDO XFET® 5.0V Voltage Reference w/Current Sink and Source 
ADR445ARMZ ADR445ARZ 
ADR01  Ultracompact, Precision 10.0 V Voltage Reference 
ADR01AKSZREEL7 ADR01ARZ ADR01AUJZREEL7 ADR01WARZR7 