ANALOG DEVICES CN-0257 Service Manual

Circuit Note
Circuits from the Lab™ circuits from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of
room temperature. However, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices
iable for direct, indirect, special, incidental, consequential or punitive damages due to any cause
+
+
+
+
968 3 2
15 17 16
19
20
24
1
CLR
IOV
CCVCCVDD
V
REFP
V
SS
V
REFN
AGND
0V TO +10V
OUTPUT
VOLTAGE
0V TO +10V
DGND
V
OUT
R
FB
7
LDAC
11
SDO
14
INV
13
SCLK
SYNC
AD5790
AD8675
AD8675
12
SDIN
4
RESET
+15V
+10V
–15V
–15V
C1 10µF
A1
A2
10µF
+3.3V +15V
+15V
–15V
0.1µF
10µF
0.1µF
10µF
0.1µF
R1
1.5kΩ
TO HIGH PRECISION
+5V REFERENCE
R3
1kΩ
6.8kΩ
6.8kΩ
SPI INTERFACE
AND DIGITAL
COTNROL
R2
1kΩ
10394-001
Circuits from the Lab™ reference circuits are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0257.
20-Bit, Linear, Low Noise, Precision, Unipolar +10 V DC Voltage Source

EVALUATION AND DESIGN SUPPORT

Circuit Evaluation Boards
AD5790 Circuit Evaluation Board (EVAL-AD5790SDZ) System Demonstration Platform (EVAL-SDP-CB1Z)
Design and Integration Files Schematics, Layout Files, Bill of Materials

CIRCUIT FUNCTION AND BENEFITS

The circuit, shown in Figure 1, is a 20-bit, linear, low noise, precision, unipolar (+10 V) voltage source with a minimum amount of external components. The AD5790 DAC is a 20-bit, unbuffered voltage output DAC that operates from a bipolar
CN-0257
Devices Connected/Referenced
AD5790 True 20-Bit, Voltage Output DAC
AD8675
AD8676
supply of up to 33 V. The AD5790 accepts a positive reference input range of 5 V to VDD − 2.5 V, and a negative reference input range of VSS + 2.5 V to 0 V. Both reference inputs are buffered on the chip, and external buffers are not required. The AD5790 offers a relative accuracy specification of ±2 LSB maximum, and operation is guaranteed monotonic with a −1 to +2 LSB maximum DNL specification.
The AD8675 precision op amp has low offset voltage (75 µV maximum) and low noise (2.8 nV/√Hz typical) and is an optimum output buffer for the AD5790. The AD5790 has two internal matched 6.8 kΩ feedforward and feedback resistors, which can either be connected to the AD8675 op amp to
Ultraprecision, 36 V, 2.8 nV√Hz, Rail-to-Rail Output Op Amp
Ultraprecision, 36 V, 2.8 nV√Hz, Dual Rail-to-Rail Output Op Amp
Rev.0
each circuit, and their function and performance have been tested and verified in a lab environment at
be l whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page)
Figure 1. 20-Bit Accurate, 0 V to +10 V Voltage Source (Simplified Schematic: All Connections and Decoupling Not Shown)
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 Fax: 781.461.3113 ©2011 Analog Devices, Inc. All rights reserved.
www.analog.com
–2.0
–1.5
–1.0
–0.5
0
0.5
1.0
1.5
2.0
0 200000 400000 600000 800000 1000000
INL (LSB)
DAC CODE
10394-002
–1.0
–0.5
0
0.5
1.5
1.0
0 200000 400000 600000 800000 1000000
DNL (LSB)
DAC CODE
10394-003
CN-0257 Circuit Note
provide a 10 V offset voltage for a ±10 V output swing, or connected in parallel to provide bias current cancellation. In this example, a unipolar +10 V output is demonstrated, and the resistors are used for bias current cancellation. The internal resistor connection is controlled by setting a bit in the AD5790 control register (see AD5790 data sheet).
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 compact circuit offers high precision, as well as low noise—this is ensured by the combination of the AD5790 and AD8675 precision components.

CIRCUIT DESCRIPTION

The digital-to-analog converter (DAC) shown in Figure 1 is the
AD5790, a high voltage, 20-bit converter with SPI interface,
offering ±2 LSB INL, −1 to +2 LSB DNL, and 8 nV/√Hz noise spectral density. The AD5790 also exhibits an extremely long term linearity error stability of 0.1 LSB.
Figure 1 shows the AD5790 in a unipolar buffered configuration. The output buffer is the AD8675, 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 precision reference, in this case a Krohn Hite Model 523 precision reference. The resistors R2 and R3 in this gain circuit are precision metal foil resistors with 0.01% tolerance and a temperature coefficient of 0.6 ppm/°C. For optimum performance over temperature, R2 and R3 should be in a single package, such as the Vishay 300144 or VSR144 series. R2 and R3 are selected to be 1 k 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.

Linearity Measurements

The precision performance of the circuit shown in Figure 1 is demonstrated on the EVAL -AD5790SDZ evaluation board using an Agilent 3458A multimeter. Figure 2 shows that the integral nonlinearity as a function of DAC code is well within the specification of ± 2 LSB from 0°C to 105°C.
Figure 3 shows that the differential nonlinearity as a function of DAC code is within the −1 LSB to +2 LSB specification.

Noise Drift Measurements

To be able to realize high precision, the peak-to-peak noise at the circuit output must be maintained below 1 LSB, which is
9.5 µV for 20-bit resolution and a +10 V unipolar voltage range. A real-time noise application 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. With this in mind, the measured peak-to­peak noise is shown in output of the circuit was measured over a period of 100 seconds, effectively including frequencies as low as 0.01 Hz in the measurement.
A temperature controlled ultralow noise reference was required for this measurement so as not to dominate the noise performance.
Figure 2. Integral Nonlinearity vs. DAC Code
Figure 3. Differential Nonlinearity vs. DAC Code
Figure 4. In this case, the noise at the
Rev. 0 | Page 2 of 6
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