CN-0177: 18-Bit, Linear, Low Noise, Precision Bipolar ±10 V DC Voltage Source

Circuit Note
CN-0177
Devices Connected/Referenced
Circuits from the Lab™ tested circuit designs address
common design challenges and are engineered for
quick and easy system integration. For more information
and/or support, visit www.analog.com/CN0177.
AD5781
True 18-Bit, Voltage Output DAC
AD8676
Ultraprecision, 36 V, 2.8 nV√Hz, Dual,
Rail-to-Rail Output Op Amp
ADR445
Ultralow Noise LDO XFET® Voltage
Reference
18-Bit, Linear, Low Noise, Precision Bipolar ±10 V DC Voltage Source
dependent and will lead to linearity errors if the DAC reference
is not adequately buffered. With a high open-loop gain of 120 dB,
the AD8676 has been proven and tested to meet the settling
time, offset voltage, and low impedance drive capability
required by this circuit application. The AD5781 is
characterized and factory calibrated using the AD8676
dual op amp to buffer its voltage reference inputs, further
enhancing confidence in partnering the components .
CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 provides a programmable 18-bit
voltage with an output range −10 V to +10 V, ±0.5 LSB integral
nonlinearity, ±0.5 LSB differential nonlinearity, and low noise.
The digital input to the circuit is serial and is compatible with
standard SPI, QSPI, MICROWIRE®, and DSP interface standards.
For high accuracy applications, the circuit offers high precision,
as well as low noise—this is ensured by the combination of the
AD5781, ADR445 and AD8676 precision components.
This combination of parts provides industry-leading 18-bit
integral nonlinearity (INL) of ±0.5 LSB and differential
nonlinearity (DNL) of ±0.5 LSB, with guaranteed monotonicity,
as well as low power, small PCB area, and cost effectiveness.
The reference buffer is critical to the design because the
input impedance at the DAC reference input is heavily code
+
+15V
10µF
ADR445
+15V
R1
1.5kΩ
0.1µF
C1 +
10µF
GND
–
1/2
AD8676
+
A1
10µF
10µF
+
−15V
R3
1kΩ
0.1µF
8 LDAC
+ –
B1
−15V
0.1µF
5
VCC
9
3
20
RFB
4
–
INV 1
11 SDO
A2
VOUT 2
AD5781
14 SYNC
VREFNF
VREFNS
AGND
6 RESET
VSS
12 SDIN
+
–10V TO +10V
OUTPUT
VOLTAGE
1/2
AD8676
13 SCLK
DGND
SPI INTERFACE
AND DIGITAL
CONTROL
IOVCC
10
7 CLR
AD8676
+15V
VDD
R2
1kΩ
1/2
+3.3V +15V
VREFPS
VOUT
VREFPF
VIN
+
0.1µF
15
18
16
17
19
0.1µF
–15V
B2
+ –
1/2
AD8676
09313-001
+
10µF
Figure 1. 18-bit Accurate, ±10 V Voltage Source (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev. 0
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CN-0177
Circuit Note
CIRCUIT DESCRIPTION
Linearity Measurements
The precision performance of the circuit shown in Figure 1 is
demonstrated in the data in Figure 2 and Figure 3, which show
integral nonlinearity and differential nonlinearity as a function
of DAC code. As can be seen, both are significantly within the
specifications of ±0.5 LSB and ±0.5 LSB, respectively.
The total unadjusted error for the circuit consists of the dc
errors combined together—that is, INL error, offset error, and
gain error. Figure 4 shows a plot of total unadjusted error as a
function of DAC code. The maximum errors occur at DAC
code zero and DAC code 262,143. This is expected, and due to
the absolute error in the voltage reference output, the mismatch
in external resistors R2 and R3 (see Figure 1), and the mismatch
in the AD5781 internal resistors RFB and R1 (see Figure 5).
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
20
TOTAL UNADJUSTED ERROR (LSB)
0.8
0.2
0
–0.2
–0.4
–0.6
–0.8
100000
150000
200000
250000
15
10
5
0
–5
–10
–15
0
50000
100000
150000
200000
DAC CODE
250000
–25
0
50000
100000
150000
200000
DAC CODE
Figure 2. Integral Nonlinearity vs. DAC Code
Figure 4. Total Unadjusted Error vs. DAC Code
Rev. 0 | Page 2 of 4
250000
09313-004
–20
09313-002
–1.0
50000
Figure 3. Differential Nonlinearity vs DAC Code
25
0.4
0
DAC CODE
1.0
0.6
INL ERROR (LSB)
1.0
09313-003
Figure 1 shows the AD5781 configured in a gain-of-two mode
such that a single reference source can be used to generate a
symmetrical bipolar output voltage range. This mode of
operation uses an external op amp (A2), as well as on-chip
resistors (see AD5781 data sheet) to provide the gain of two.
These internal resistors are thermally matched to each other
and to the DAC ladder resistance, resulting in ratiometric
thermal tracking. The output buffer is again the AD8676, used
for its low noise and low drift. This amplifier is also used (A1)
to amplify the +5 V reference voltage from the low noise
ADR445 to +10 V. R2 and R3 in this gain circuit are precision
metal foil resistors with 0.01% tolerance and a temperature
coefficient resistance of 0.6 ppm/°C. For optimum performance
over temperature, R1 and R2 should be in a single package, such
as the Vishay 300144 or VSR144 series. R2 and R3 are selected
to be 1 kΩ to keep noise in the system low. R1 and C1 form a
low-pass filter with a cutoff frequency of approximately 10 Hz.
The purpose of this filter is to attenuate voltage reference noise.
DNL ERROR (LSB)
The digital-to-analog converter (DAC) shown in Figure 1 is the
AD5781, a high voltage, 18-bit converter with SPI interface,
offering ±0.5 LSB INL, ±0.5 LSB DNL, and 7.5 nV/√Hz noise
spectral density. The AD5781 also exhibits an extremely low
temperature drift of 0.05 ppm/°C. The precision architecture of
the AD5781 requires force-sense buffering of its voltage
reference inputs to ensure specified linearity. The amplifiers (B1
and B2) chosen to buffer the reference inputs should 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 low
offset drift of 0.6 µV/°C and input bias currents of 2 nA. In
addition, the AD5781 is characterized and factory calibrated
using this dual op amp to buffer its voltage reference inputs,
further enhancing confidence in partnering the components .
Circuit Note
CN-0177
A real application, however, will not have a high-pass cutoff at
0.1 Hz to attenuate 1/f noise, but will include frequencies down
to dc in its pass band; therefore, the measured peak-to-peak
noise will be more realistically shown in Figure 7. In this case,
the noise at the output of the circuit was measured over a period
of 100 seconds, effectively including frequencies as low as
0.01 Hz in the measurement. The upper frequency cutoff is at
approximately 14 Hz and is limited by the measurement setup.
For the three conditions shown in Figure 7, the peak-to-peak
values are 1.61 µV for mid-scale output, 43.33 µV for full-scale
output, and 36.89 µV for zero-scale output. The worst-case peakto-peak value of 43.33 µV corresponds to approximately ½ LSB.
VREFP = +10V
B1
VREFPS
R1
RFB
RFB
6.8kΩ 6.8kΩ
INV
18-BIT
DAC
VREFNF
VOUT
A2
1/2
AD8676
VOUT
AD5781
VREFNS
25
15
10
Figure 5. Internal Gain-of-Two Circuitry (Simplified Schematic)
The specified voltage reference absolute error is 0.04%; the
specified mismatch in resistors R2 and R3 in this case is 0.02%;
the specified mismatch in internal resistors R1 and RFB is 0.01%.
This results in a total gain error of 0.07% of full-scale range, or
184 LSBs. Figure 4 shows the measured value to be 20 LSBs, or
0.007% of full-scale range, indicating that all components are
performing significantly better than their specified tolerances.
Noise Measurements
5
0
–5
–10
–15
ZERO-SCALE
FULL-SCALE
MID-SCALE
–20
–25
.
To be able to realize high precision, the peak-to-peak noise at
the circuit output must be maintained below 1 LSB, which is
76.29 µV for 18-bit resolution and a 20 V peak-to-peak voltage
range. Figure 6 shows peak-to-peak noise measured in the
0.1 Hz to 10 Hz bandwidth over a period of 10 seconds. The
peak-to-peak values for each of the three conditions are 1.34 µV
for mid-scale output, 12.92 µV for full-scale output, and 15.02 µV
for zero-scale output. Mid-scale output exhibits the lowest
noise, as it represents the noise from the DAC core only.
The noise contribution from each voltage reference path is
attenuated by the DAC when mid-scale code is selected.
0
20
40
60
TIME (Seconds)
80
100
Figure 7. Voltage Noise Measured Over 100 Second Period
As the time period over which the measurement is taken is
increased, lower frequencies will be included, and the peak-topeak value will increase. At low frequencies, temperature drift
and thermocouple effects become contributors to noise. These
effects can be minimized by choosing components with low
thermal coefficients. In this circuit, the main contributor to low
frequency 1/f noise is the voltage reference. It also exhibits the
greatest temperature coefficient value in the circuit of 3 ppm/°C.
COMMON VARIATIONS
–6
The AD5781 will support a wide variety of output ranges from
0 V to +5 V up to ±10 V, and values in between. The gain-oftwo configuration, as shown in Figure 1, can be used if a
symmetrical output range is required. This mode is selected by
setting the RBUF bit of the AD5781 internal control register to
a Logic 0. If an asymmetrical range is required, individual
references can be applied at VREFP and VREFN; and the output
buffer should be configured for unity gain as described in the
AD5781 data sheet. This is done by setting the RBUF bit of the
AD5781 internal control register to a Logic 1.
–8
CIRCUIT EVALUATION AND TEST
10
ZERO-SCALE
FULL-SCALE
MID-SCALE
8
6
4
2
0
–2
–4
–10
0
2
4
6
8
10
TIME (Seconds)
Figure 6. Voltage Noise in 0.1 Hz to 10 Hz Bandwidth
09313-006
OUTPUT VOLTAGE (µV)
20
09313-005
1/2 AD8676 B2
09313-007
VREFPF
VOLTAGE (µV)
1/2 AD8676
The circuit shown in Figure 1 was constructed on a modified
AD5781 evaluation board. Details of the AD5781 evaluation
board and test methods can be found in Evaluation Board
User Guide UG-184.
Rev. 0 | Page 3 of 4
CN-0177
Circuit Note
LEARN MORE
Egan, Maurice. "The 20-Bit DAC Is the Easiest Part of a 1-ppmAccurate Precision Voltage Source," Analog Dialogue,
Vol. 44, April 2010.
Kester, Walt. 2005. The Data Conversion Handbook. Analog
Devices. Chapters 3 and 7.
MT-015 Tutorial, Basic DAC Architectures II: Binary DACs.
Analog Devices.
MT-016 Tutorial, Basic DAC Architectures III: Segmented DACs.
Analog Devices.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of AGND and DGND. Analog Devices.
MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and
Rail-to-Rail Issues. Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
Voltage Reference Wizard Design Tool.
Data Sheets and Evaluation Boards
AD5781 Data Sheet
AD5781 Evaluation Board
AD8676 Data Sheet
ADR445 Data Sheet
REVISION HISTORY
1/11—Revision 0: Initial Version
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Rev. 0 | Page 4 of 4
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