Download CIRCUIT FUNCTION AND BENEFITS

Figure 1 shows that the differential ac-coupled source has
signals 180° out of phase with respect to each other, and the
voltage swings around ground on each input. In the test setup,
an Audio Precision AP2700-series generator was used to
generate the differential input signals. Two 10 µF NP0
capacitors are used to couple the signal into the driver circuit.
The two ADA4899-1 amplifiers are connected in a unity gain
noninverting configuration (ADA4899-1 is unity gain stable) to
condition the analog input to the AD7625 inputs, thereby
providing sufficient isolation from the converter switched
capacitor transients and also setting the correct common-mode
input voltage. For the AD7625, the common-mode voltage
is one-half the internal reference voltage, REF/2, where
REF = 4.096 V.
CIRCUIT FUNCTION AND BENEFITS
This circuit provides a method to drive an ac-coupled
differential input signal to the AD7625, 16-bit, 6 MSPS PulSAR®
differential ADC. This circuit has been designed to ensure
maximum performance of the AD7625 by providing adequate
settling time and low distortion. It uses a buffered VCM output
voltage from the AD7625 to set each amplifier’s common-mode
level.
CIRCUIT DESCRIPTION
The signal source applied to the AD7625 should be buffered to
enable driving the AD7625 switch capacitor front end and
maintain low distortion. The ADA4899-1 used on each input
provides the required drive, distortion, and settling time to
maximize the performance of the AD7625 16-bit, 6 MSPS ADC.
+7V
+REF/2
10µF, NP0
0
0.1µF
49.9Ω
33Ω
590Ω
+5V
ADA4899-1
FB
–REF/2
0.1µF
0.1µF
VDD1 VDD2
IN+
−5V
IN−
+7V
10µF, NP0
+REF/2
AD7625
VCM
GND
33Ω
ADA4899-1
FB
0
+7V
56pF
0.1µF
0.1µF
0.1µF
–REF/2
VIO
0.1µF
49.9Ω
590Ω
+2.5V +2.5V
56pF
REF = 4.096V
VCM = REF/2
+2.048V
−5V
10µF
1MΩ
08316-001
AD8031
Figure 1. AC-Coupled Differential Drive Circuit for the AD7625 ADC (Simplified Schematic: All Connections and Decoupling Not Shown)
www.BDTIC.com/ADI
The VCM output pin of the AD7625 is a useful function that
provides one-half the reference voltage used internally by the
AD7625. This VCM output is buffered with the AD8031 rail-torail amplifier, thereby providing a precise common-mode
voltage for the analog input amplifiers.
Excellent layout, grounding, and decoupling techniques must be
utilized in order to achieve the desired performance from the
circuits discussed in this note (see Tutorial MT-031 and Tutorial
MT-101). As a minimum, a 4-layer PCB should be used with
one ground plane layer, one power plane layer, and two signal
layers. The AD7625 data sheet also includes a section on layout
and decoupling practices for the device.
Figure 2 and Figure 3 show the excellent distortion and noise
performance obtained with the circuit.
0
–60
–80
–100
–120
–140
–160
–180
0
5
10
15
20
25
30
35
40
45
FREQUENCY (kHz)
Figure 3. Expanded View of the FFT with a 2 kHz Input Tone and a Sampling
Rate of 6 MSPS
LEARN MORE
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND", Analog Devices.
MT-036 Tutorial, Op Amp Output Phase-Reversal and Input
Over-Voltage Protection, Analog Devices.
MT-074 Tutorial, Differential Drivers for Precision ADCs,
Analog Devices.
MT-075 Tutorial, Differential Drivers for High Speed ADCs
Overview, Analog Devices.
–20
AMPLITUDE (dB)
–40
08316-003
In order to allow sufficient headroom at the output of the
ADA4899-1 op amps when they go to +4.096 V and 0 V, both
devices are powered with a +7 V and −5 V supply. Because the
amplifier and the ADC operate on different supply voltages,
protection circuits may be required at the ADC inputs as
described in Tutorial MT-036.
INPUT TONE = 2kHz
SNR = 93.16dB
SINAD = 92.09dB
THD = –110.45dB
SFDR = 111.37dB
–20
AMPLITUDE (dB)
Note that the feedback pin (FB) on the ADA4899-1 is internally
connected to its output pin, thereby minimizing parasitic
capacitance and inductance. The ADA4899-1 also has an
exposed pad for heat dissipation, which should be electrically
connected to the ground plane.
0
–40
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
–60
Data Sheets and Evaluation Boards
–80
AD7625 Data Sheet
–100
AD7625 Evaluation Board
–120
AD8031 Data Sheet
–140
ADA4899-1 Data Sheet
–180
0
0.5
1.0
1.5
2.0
2.5
3.0
FREQUENCY (MHz)
Figure 2. FFT Output with a 2 kHz Input Tone and a Sampling
Rate of 6 MSPS
08316-002
–160
REVISION HISTORY
9/09—Revision 0: Initial Version
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