AD8051/AD8052/AD8054
CLK
SLEEP
INA-I
0.33F
0.01F
22⍀
؉V
SELECT
DD
+5V
10pF
1k⍀
22⍀
INB-I
10pF
10F
0.1F
AD9201
DATA OUT
REFT-I
D9
D8
D7
D6
D5
D4
D3
D2
22⍀
10F
0.1F
0.1F
0.1F
10F
REFB-I
50⍀
AD8051
AVSS
REFSENSE
1k⍀
1k⍀
V
REF
0.1F
10F
0.1F
؊5V
AVDD
؉5V
10F
0.1F
REFB-Q
REFT-Q
0.1F
10F
0.1F
D1
D0
0.1F
22⍀
؉5V
DVDD
DVSS
INB-Q
10pF
0.1F
10F
22⍀
INA-Q
10pF
THREE–STATE
Figure 44. AD8051 Driving an AD9201, a 10-Bit 20 MSPS A/D Converter
applied to the noninverting input of the AD8051. The amplifier
output is 2 V p-p, which is the maximum input range of the
AD9201. The 22 Ω series resistor limits the maximum current
that flows and helps to lower the distortion of the A/D.
positive and negative with respect to the bias voltage applied to
INB-I.
With the sampling clock running at 20 MSPS, the A/D 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.
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 8) 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.
Figure 45 shows the FFT response of the A/D for the case of
1 MHz analog input. The SFDR is 71.66 dB and the A/D is
producing 8.8 ENOB (effective number of bits). When the
analog frequency was raised to 9.5 MHz, the SFDR was re-
duced to 60.18 dB and the A/D operated with 8.46 ENOBs as
shown in Figure 46. The inclusion of the AD8051 in the circuit
had no worsening of the distortion performance of the AD9201.
The output of the op amp is ac coupled into INA-I (Pin 2) via
two parallel capacitors to provide good high frequency and low
frequency coupling. The 1 kΩ resistor references the signal to
VREF that is applied to INB-I. Thus, INA-I will swing both
10.0
10.0
PART#
FFTSIZE 8192
0
PART#
0
5.0
0.0
5.0
0.0
FUND
FUND
FFTSIZE 8192
FCLK
FUND
VIN
20.0E؉6
؊5.0
؊10.0
؊15.0
FCLK
FUND
20.0E؉6
؊5.0
9.5E؉6
؊10.0
؊15.0
؊20.0
؊25.0
؊30.0
؊35.0
؊40.0
؊45.0
؊50.0
؊55.0
؊60.0
؊65.0
؊70.0
؊75.0
؊80.0
؊85.0
؊90.0
؊95.0
؊100.0
؊105.0
؊110.0
؊115.0
؊120.0
998.5E؉3
؊0.51dB
؊68.13
54.97
؊0.44dB
VIN
؊20.0
؊25.0
؊30.0
THD
؊57.08
54.65
THD
SNR
SNR
SINAD
ENOB
SFDR
2ND
52.69
SINAD
ENOB
SFDR
54.76
؊35.0
؊40.0
؊45.0
؊50.0
8.46
8.80
؊60.18
؊60.18
؊71.66
؊74.53
؊76.06
؊76.35
؊79.05
؊80.36
2ND
3RD
؊55.0
؊60.0
؊65.0
3RD
؊60.23
؊82.01
؊78.83
؊81.28
؊77.28
؊84.54
2ND
3RD
4TH
5TH
4TH
5TH
؊70.0
؊75.0
؊80.0
؊85.0
7TH
6TH
7TH
8TH
9TH
2ND
9TH
7TH
4TH
6TH
7TH
3RD
8TH
6TH
6TH
4TH
5TH
؊75.08
؊88.12
؊77.87
8TH
8TH
9TH
؊90.0
؊95.0
؊92.78
؊100.0
؊105.0
؊110.0
؊115.0
؊120.0
0.0E؉0
2.0E؉6
4.0E؉6
6.0E؉6
8.0E؉6
10.0E؉6
9.0E؉6
0.0E؉0
2.0E؉6
4.0E؉6
6.0E؉6
8.0E؉6
10.0E؉6
9.0E؉6
1.0E؉6
3.0E؉6
5.0E؉6
7.0E؉6
1.0E؉6
3.0E؉6
5.0E؉6
7.0E؉6
Figure 46. FFT Plot for AD8051 Driving the AD9201 at
9.5 MHz
Figure 45. FFT Plot for AD8051 Driving the AD9201 at
1 MHz
–14–
REV. B
ADI [ ADI ]
