ANALOG DEVICES CN-0237 Service Manual

Circuit Note

e 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

ect, special, incidental, consequential or punitive damages due to any cause

R5

10kΩ

V

IN+

V

OCM

V

IN–

R3

1kΩ

+IN

+6.0V

+6.0V

–1.0V

–IN

SERIAL

INTERFACE

–OUT

+OUT

R4

1kΩ

R2

1kΩ

33Ω

33Ω

+5V

+2.5V

ADR395

2.7nF

2.7nF

AD7982

ADA4940-1

IN+

IN–

REF VDD

GND

R1

1kΩ

R6

10kΩ

0.1µF

10µF

+2.5V

10144-001

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/CN0237.

Ultralow Power, 18-Bit, Differential PulSAR ADC Driver

EVALUATION AND DESIGN SUPPORT

Design and Integration Files
Schematics, Layout Files, Bill of Materials

CIRCUIT FUNCTION AND BENEFITS

The circuit, shown in Figure 1, uses the ultralow power AD7982
18-bit, 1 MSPS ADC driven by the ADA4940-1, a low power
fully differential amplifier. The ADR395, low noise precision 5.0
V voltage reference is used to supply the 5 V needed for the
ADC. All the ICs shown in Figure 1 are available in small
packages, either 3 mm × 3 mm LFCSP, or 3 mm × 5 mm MSOP,
which helps reduce board cost and space.

Power dissipation of the ADA4940-1 in the circuit is less than
9 mW. The 18-bit, 1 MSPS AD7982 ADC consumes only 7 mW
@ 1 MSPS, which is much lower than competitive ADCs
available in the market. This power also scales with the
throughput. The ADR395 consumes only 0.7mW, making the
total power dissipated by the system less than 17 mW.

CN-0237

Devices Connected/Referenced

ADA4940-1/
ADA4940-2

AD7982 18-Bit, 1 MSPS PulSAR ADC

ADR395

CIRCUIT DESCRIPTION

Modern high resolution SAR ADCs, such as the AD7982 18-bit,
1 MSPS PulSAR® ADC, require a differential driver for optimum
performance. In such applications, the ADC driver takes either
a differential or single-ended signal and performs the level
shifting required to drive the input of the ADC at the right level.

Figure 1 shows the ADA4940-1 differential amplifier level
shifting and driving the 18-bit AD7982 differential input
successive approximation PulSAR ADC. Using four resistors,
the ADA4940-1 can either buffer the signal with a gain = 1 or
amplify the signal for more dynamic range. The ac and dc
performances are compatible with those of the 18-bit, 1 MSPS

AD7982 PulSAR® ADC and other 16- and 18-bit members of

the family, which have sampling rates up to 2 MSPS. This circuit
can also accept a single-ended input signal to generate the same
fully differential output signal.

Single/Dual, Ultralow Power, Low
Distortion Differential ADC Driver

Micropower, Low Noise, Precision 5 V
Bandgap Voltage Reference

Rev.0

Circuits from th

each circuit, and their function and performance have been tested and verified in a lab environ ment at

be liable for direct, indir
whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page)

Figure 1. High Performance 18-Bit Differential ADC Driver (Simplified Schematic: All Connections and Decoupling Not Shown)

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
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Fax: [phone] ©2011 Analog Devices, Inc. All rights reserved.

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CN-0237 Circuit Note

10144-002

The AD7982 operates on a single VDD supply of 2.5 V. It
contains a low power, high speed, 18-bit sampling ADC and a
versatile serial interface port. The reference voltage (REF) is
applied externally from the ADR395 precision low dropout
(0.3 V) band gap reference, and can be set independently of the
supply voltage. The ADA4940-1 is dc coupled on the input and
the output and performs a differential or single-ended-to­differential conversion if needed. It also buffers the driving
signal. A single-pole 1.8 MHz R-C (33 Ω, 2.7 nF) noise filter is
placed between the op amp output and the ADC input. The
filter also provides some isolation between the op amp output
and the switching spikes at the ADC input due to the internal
sample-and-hold function.

The ADA4940-1 is driven with a 7 V supply (+6 V and –1 V) in
order to provide sufficient headroom on the outputs, which must
swing from 0 V to +5 V for a full-scale input to the ADC.

The gain is set by the ratio of the feedback resistor (R2 = R4) to
the gain resistor (R1 = R3). In addition, the circuit can be used
to convert either single-ended or differential inputs to a differential
output. If needed, a termination resistor in parallel with the
input can be used. Whether the input is a single-ended input or
differential input, the input impedance of the amplifier can be
calculated as show in the MT-076 Tutorial and in the DiffAmpCalc™
Differential Amplifier Calculator (www.analog.com/diffampcalc).

If R1 = R2 = R3 = R4 = 1 kΩ, the single-ended input impedance
is approximately 1.33 kΩ. An external 52.3 Ω termination
resistor provides a 50 Ω termination for the source. An
additional 25.5 Ω (1025.5 Ω total) at the inverting input
balances the parallel impedance of the 50 Ω source and
the termination resistor driving the noninverting input
(52.3 Ω || 50 Ω = 25.5 Ω). Howe ve r, if a differential source input
is used, the differential input impedance is 2 kΩ. In this case, two

52.3 Ω termination resistors are used to terminate each input
if needed.

Figure 2. FFT Plot (32,000 Point) for 20 kHz Signal, 0.5 dB Below Full Scale, with Sampling Frequency of 1 MSPS

Rev. 0 | Page 2 of 5