Datasheet

AD8051/AD8052/AD8054
Rev. J | Page 19 of 24
ANALOG-TO-DIGITAL AND DIGITAL-TO-ANALOG
APPLICATIONS
Figure 50 is a schematic showing the AD8051 used as a driver
for an AD9201, a 10-bit, 20 MSPS, dual analog-to-digital
converter. This converter is designed to convert I and Q signals in
communications systems. In this application, only the I channel
is being driven. The I channel is enabled by applying a logic
high to SELECT (Pin 13).
The AD8051 is running from a dual supply and is configured
for a gain of +2. The input signal is terminated in 50  and the
output is 2 V p-p, which is the maximum input range of the
AD9201. The 22  series resistor limits the maximum current
that flows and helps to lower the distortion of the ADC.
The AD9201 has differential inputs for each channel. These are
designated the A and B inputs. The B inputs of each channel are
connected to VREF (Pin 22), which supplies a positive reference
of 2.5 V. Each of the B inputs has a small low-pass filter that also
helps to reduce distortion.
The output of the op amp is ac-coupled into INA-I (Pin 16) via
two parallel capacitors to provide good high frequency and low
frequency coupling. The 1 k resistor references the signal to
VREF that is applied to INB-I. Thus, INA-I swings both positive
and negative with respect to the bias voltage applied to INB-I.
With the sampling clock running at 20 MSPS, the analog-to-
digital output was analyzed with a digital analyzer. Two input
frequencies were used, 1 MHz and 9.5 MHz, which is just short
of the Nyquist frequency. These signals were well filtered to
minimize any harmonics.
Figure 48 shows the FFT response of the ADC for the case of a
1 MHz analog input. The SFDR is 71.66 dB, and the analog-to-
digital is producing 8.8 ENOB (effective number of bits). When
the analog frequency was raised to 9.5 MHz, the SFDR was
reduced to −60.18 dB and the ADC operated with 8.46 ENOBs
as shown in Figure 49. The inclusion of the AD8051 in the
circuit did not worsen the distortion performance of the AD9201.
PART# 0
FCLK
FUND
VIN
THD
SNR
SINAD
ENOB
SFDR
2ND
3RD
4TH
5TH
6TH
7TH
8TH
9TH
FFTSIZE 8192
20.0MHz
998.5kHz
–0.51dB
–68.13
54.97
54.76
8.80
71.66
–74.53
–76.06
–76.35
–79.05
–80.36
–75.08
–88.12
–77.87
10
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
AMPLITUDE (dB)
FREQUENCY (MHz)
0 12 34567 8910
FUND
2ND
5TH
6TH
7TH
8TH
9TH
4TH3RD
01062-049
Figure 48. FFT Plot for AD8051 Driving the AD9201 at 1 MHz
PART#
0
FCLK
FUND
VIN
THD
SNR
SINAD
ENOB
SFDR
2ND
3RD
4TH
5TH
6TH
7TH
8TH
9TH
FFTSIZE 8192
20.0MHz
9.5MHz
–0.44dB
–57.08
54.65
52.69
8.46
60.18
–60.18
–60.23
–82.01
–78.83
–81.28
–77.28
–84.54
–92.78
10
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
AMPLITUDE (dB)
FREQUENCY (MHz)
0 1234567 8910
FUND
2ND
5TH
6TH
7TH
8TH
4TH
3RD
01062-050
Figure 49. FFT Plot for AD8051 Driving the AD9201 at 9.5 MHz
AD8051
+5V
VREF
AVDD
SELECT
INA-I
10pF
CLOCK
SLEEP
D9
D1
D2
D3
D4
D5
D6
D7
D0
DVDD
AVSS
REFSENSE
AD9201
DVSS
CHIP–SELECT
INB-I
REFT-I
REFB-I
REFB -Q
REFT -Q
INB-Q
INA-Q
D8
DATA OUT
10pF
–5V
10pF
10pF
15
16
17
18
19
20
21
22
23
24
25
26
27
28
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0.1µF 10µF
+5V
+V
DD
10µF0.1µF 0.1µF
0.1µF
10µF
0.1µF
10µF
0.1µF
10µF0.1µF 0.1µF
0.1µF
+5V
22Ω
22Ω
22Ω
22Ω
22Ω
1kΩ
1kΩ
0.33µF
0.01µF
1kΩ
10µF0.1µF
10µF0.1µF
50Ω
3
2
7
4
6
01062-048
Figure 50. The AD8051 Driving an AD9201, a 10-Bit, 20 MSPS Analog-to-Digital Converter