Transcript
Circuit Note
CN-0026
Devices Connected/Referenced Circuit Designs Using Analog Devices Products Apply these product pairings quickly and with confidence. For more information and/or support call 1-800-AnalogD (1-800-262-5643) or visit www.analog.com/circuit.
AD5547/ AD5557
Dual, Current Output, Parallel Input, 16-/14-Bit DAC
ADR03
Precision 2.5 V Voltage Reference
AD8628
Rail-to-Rail Input/Output Operational Amplifier
Precision, Unipolar, Inverting Conversion Using the AD5547/AD5557 DAC CIRCUIT FUNCTION AND BENEFITS This circuit provides precision, unipolar, inverting data conversion using the AD5547/AD5557 current output DAC with the ADR03 precision reference and AD8628 operational amplifier (op amp). This circuit provides accurate, low noise, high speed output voltage capability and is well suited for process control, automatic test equipment, and digital calibration applications.
CIRCUIT DESCRIPTION The AD5547/AD5557 are dual-channel, precision 16-/14-bit, multiplying, low power, current output, parallel input digital-toanalog converters. They operate from a single 2.7 V to 5.5 V supply with ±15 V multiplying references for 4-quadrant outputs. Built-in 4-quadrant resistors facilitate the resistance
+5V C1 1µF
matching and temperature tracking that minimize the number of components needed for multiquadrant applications. This circuit uses the ADR03, which is a highly accuracy, high stability, 2.5 V precision voltage reference. As voltage reference temperature coefficient and long-term drift are primary considerations for applications requiring high precision conversion, this device is an ideal candidate. An op amp is used in the current-to-voltage (I-V) 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 AD8628 autozero op amp, which has ultralow offset voltage (1 µV typical) and bias current (30 pA typical). The compensation capacitor, C7, is optimized to compensate for the external output capacitance of the DAC.
2 C2 0.1µF
U3 ADR03 VIN TRIM VOUT
5 +2.5V
6
C4 0.1µF
GND
VREFA 4
RCOMA
R1A VDD
C3 0.1µF
R1
ROFS
R2
2.5V 16-BIT/ 14-BIT
AD5547/AD5557
16-BIT/ 14-BIT DATA
U1 WR LDAC RS
RFBA
C7 2.2pF
RFB IOUTA AGNDA
VOUTA
+V
AD8628 –V
–2.5V TO 0V C5 0.1µF
MSB A0, A1
C6 1µF 2 –5V
08248-001
WR LDAC RS MSB A0, A1
ROFSA
Figure 1. Unipolar 2-Quadrant Multiplying Mode, VOUT = 0 V to –VREF (Simplified Schematic)
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CN-0026
Circuit Note
Note that the AD8628 has rail-to-rail input and output stages, but the output can only come within a few millivolts of either rail depending on load current. For the circuit shown, the output can swing from –2.5 V to approximately –1 mV. The input offset voltage of the op amp is multiplied by the variable noise gain (due to the code-dependent 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 ADR03 and the AD5547, the LSB size is
2. 5 V
= 38 µV (1) 216 The input offset voltage of the AD8628 auto-zero op amp is typically 1 µV, which is negligible compared to the LSB size. 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 AD8628, the input bias current is only 30 pA typical, which flowing through the RFB resistor (10 kΩ typical), produces an error of only 0.3 µV. The AD5547/AD5557 DAC architecture uses a current-steering R-2R 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
VOUT =
−VREF × D
(2) 216 where D is the decimal equivalent of the input code. VOUT can be calculated for the AD5557 using the equation
VOUT =
−VREF × D 214
(3)
where D is the decimal equivalent of the input code.
voltage and low bias current. The ADR01 and ADR02 are other low noise references available from the same reference family as the ADR03. Other low noise references that would be suitable are the ADR441 and ADR445 products. The size of the reference input voltage is restricted by the rail-to-rail voltage of the op amp selected. These circuits can also be used as a variable gain element by utilizing the multiplying bandwidth nature of the R-2R 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.
LEARN MORE ADIsimPower Design Tool. Kester, Walt. 2005. The Data Conversion Handbook. Analog Devices. See chapters 3 and 7. MT-015 Tutorial, Basic DAC Architectures II: Binary DACs. Analog Devices. MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of AGND and DGND. Analog Devices. MT-033 Tutorial, Voltage Feedback Op Amp Gain and Bandwidth. Analog Devices. MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and Rail-to-Rail Issues. Analog Devices. MT-055 Tutorial, Chopper Stabilized (Auto-Zero) Precision Op Amps. Analog Devices. MT-101 Tutorial, Decoupling Techniques. Analog Devices.
Data Sheets AD5547 Data Sheet. AD5557 Data Sheet. AD8628 Data Sheet. ADR03 Data Sheet.
REVISION HISTORY
COMMON VARIATIONS For multichannel applications, the AD8629 is a dual version of the AD8628. The AD8605 is another excellent op amp candidate for the I-V conversion circuit. It also has a low offset
5/09—Rev. 0 to Rev. A Updated Format .................................................................. Universal 10/08—Revision 0: Initial Version
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