AD9221/AD9223/AD9220
The addition of a differential input structure gives the user an
additional level of flexibility that is not possible with traditional
flash converters. The input stage allows the user to easily con-
figure the inputs for either single-ended operation or differential
operation. The A/D’s input structure allows the dc offset of the
input signal to be varied independently of the input span of the
converter. Specifically, the input to the A/D core is the differ-
ence of the voltages applied at the VINA and VINB input pins.
Therefore, the equation,
The SHA’s optimum distortion performance for a differential or
single-ended input is achieved under the following two condi-
tions: (1) the common-mode voltage is centered around mid
supply (i.e., AVDD/2 or approximately 2.5 V) and (2) the input
signal voltage span of the SHA is set at its lowest (i.e., 2 V input
span). This is due to the sampling switches, QS1, being CMOS
switches whose RON resistance is very low but has some signal
dependency which causes frequency dependent ac distortion
while the SHA is in the track mode. The RON resistance of a
CMOS switch is typically lowest at its midsupply but increases
symmetrically as the input signal approaches either AVDD or
AVSS. A lower input signal voltage span centered at midsupply
reduces the degree of RON modulation.
VCORE = VINA – VINB
(1)
defines the output of the differential input stage and provides
the input to the A/D core.
The voltage, VCORE, must satisfy the condition,
Figure 32a compares the AD9221/AD9223/AD9220’s THD vs.
frequency performance for a 2 V input span with a common-
mode voltage of 1 V and 2.5 V. Note how each A/D with a
common-mode voltage of 1 V exhibits a similar degradation in
THD performance at higher frequencies (i.e., beyond 750 kHz).
Similarly, note how the THD performance at lower frequencies
becomes less sensitive to the common-mode voltage. As the
input frequency approaches dc, the distortion will be domi-
nated by static nonlinearities such as INL and DNL. It is
important to note that these dc static nonlinearities are inde-
pendent of any RON modulation.
–VREF ≤ VCORE ≤ VREF
(2)
where VREF is the voltage at the VREF pin.
While an infinite combination of VINA and VINB inputs exist
that satisfy Equation 2, there is an additional limitation placed
on the inputs by the power supply voltages of the AD9221/
AD9223/AD9220. The power supplies bound the valid operat-
ing range for VINA and VINB. The condition,
AVSS – 0.3 V < VINA < AVDD + 0.3 V
AVSS – 0.3 V < VINB < AVDD + 0.3 V
(3)
where AVSS is nominally 0 V and AVDD is nominally +5 V,
defines this requirement. Thus, the range of valid inputs for
VINA and VINB is any combination that satisfies both Equa-
tions 2 and 3.
–50
AD9220
1V
CM
–60
For additional information showing the relationship between
VINA, VINB, VREF and the digital output of the AD9221/
AD9223/AD9220, see Table IV.
AD9223
1V
AD9221
CM
CM
1V
–70
–80
–90
Refer to Table I and Table II at the end of this section for a
summary of both the various analog input and reference
configurations.
AD9223
2.5V
CM
AD9220
ANALOG INPUT OPERATION
AD9221
2.5V
2.5V
CM
CM
Figure 32 shows the equivalent analog input of the AD9221/
AD9223/AD9220 which consists of a differential sample-and-
hold amplifier (SHA). The differential input structure of the
SHA is highly flexible, allowing the devices to be easily config-
ured for either a differential or single-ended input. The dc
offset, or common-mode voltage, of the input(s) can be set to
accommodate either single-supply or dual supply systems. Also,
note that the analog inputs, VINA and VINB, are interchange-
able with the exception that reversing the inputs to the VINA
and VINB pins results in a polarity inversion.
0.1
1
10
FREQUENCY – MHz
Figure 32a. AD9221/AD9223/AD9220 THD vs. Frequency for
CM = 2.5 V and 1.0 V (AIN = –0.5 dB, Input Span = 2.0 V p-p)
V
Due to the high degree of symmetry within the SHA topology, a
significant improvement in distortion performance for differen-
tial input signals with frequencies up to and beyond Nyquist can
be realized. This inherent symmetry provides excellent cancella-
tion of both common-mode distortion and noise. Also, the
required input signal voltage span is reduced by a half which
further reduces the degree of RON modulation and its effects on
distortion.
C
H
Q
S2
+
C
C
PIN
PAR
C
Q
S
S1
The optimum noise and dc linearity performance for either differ-
ential or single-ended inputs is achieved with the largest input
signal voltage span (i.e., 5 V input span) and matched input
impedance for VINA and VINB. Note that only a slight degra-
dation in dc linearity performance exists between the 2 V and
5 V input span as specified in the AD9221/AD9223/AD9220
DC SPECIFICATIONS.
VINA
VINB
Q
C
Q
H1
S
S1
–
C
C
PIN
PAR
Q
S2
C
H
Figure 32. AD9221/AD9223/AD9220 Simplified Input Circuit
–10–
REV. D
ADI [ ADI ]
