AD7722 Analog Devices, AD7722 Datasheet - Page 15

• Manufacturer Part Number: AD7722
• Description: CMOS, 16-Bit, 195 kSPS Sigma-Delta ADC
• Manufacturer: Analog Devices

Specifications of AD7722

• Resolution (bits): 16bit
• # Chan: 1
• Sample Rate: 12.5MSPS
• Interface: Par,Ser
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: (Vref) p-p,2.5V p-p,Uni (Vref),Uni 2.5V
• Adc Architecture: Sigma-Delta
• Pkg Type: QFP

APPLYING THE AD7722
Analog Input Range
The AD7722 uses differential inputs to provide common-mode
noise rejection (i.e., the converted result will correspond to the
differential voltage between the two inputs). The absolute voltage
on both inputs must lie between AGND and AV
In unipolar mode, the full-scale analog input range
(V
binary in the unipolar mode with 1 LSB = 38 µV. The ideal transfer
function is shown in Figure 11.
In bipolar mode, the full-scale input range is ± V
bipolar mode allows complementary input signals. As another
example, in bipolar mode, V
voltage to allow a single-ended input on V
± V
with 1 LSB = 38 µV. The ideal transfer function is shown in
Figure 12.
REV. B
IN
REF2
(+) – V
/2. In bipolar mode, the output code is twos complement
Figure 11. Unipolar Mode Transfer Function
Figure 12. Bipolar Mode Transfer Function
000...011
000...010
000...001
000...000
111...111
111...110
111...101
111...100
011...111
011...110
000...010
000...001
000...000
111...111
111...110
100...001
100...000
IN
(–)) is 0 V to V
OUTPUT
OUTPUT
CODE
CODE
–V
DIFFERENTIAL INPUT VOLTAGE V
DIFFERENTIAL INPUT VOLTAGE V
0V
REF2
IN
REF2
(–) can be connected to a dc bias
. The output code is straight
0V
+V
IN
IN
IN
REF2
(+) equal to V
(+) – V
(+) – V
V
REF2
/ 2 – 1LSB
REF2
DD
IN
IN
–1LSB
(–)
(–)
.
/2. The
BIAS
–15–
Differential Inputs
The analog input to the modulator is a switched capacitor design.
The analog signal is converted into charge by highly linear
sampling capacitors. A simplified equivalent circuit diagram of
the analog input is shown in Figure 13. A signal source driving
the analog input must be able to provide the charge onto the
sampling capacitors every half CLKIN cycle and settle to the
required accuracy within the next half cycle.
Since the AD7722 samples the differential voltage across its
analog inputs, low noise performance is attained with an input
circuit that provides low common-mode noise at each input.
The amplifiers used to drive the analog inputs play a critical role
in attaining the high performance available from the AD7722.
When a capacitive load is switched onto the output of an op amp,
the amplitude will momentarily drop. The op amp will try to
correct the situation and, in the process, will hit its slew rate limit.
This nonlinear response, which can cause excessive ringing, can
lead to distortion. To remedy the situation, a low-pass RC filter
can be connected between the amplifier and the input to the
AD7722 as shown in Figure 14. The external capacitor at each
input aids in supplying the current spikes created during the
sampling process. The resistor in this diagram, as well as creating
the pole for the antialiasing, isolates the op amp from the transient
nature of the load.
The differential input impedance of the AD7722 switched capacitor
input varies as a function of the CLKIN frequency, given by the
equation
ANALOG
INPUT
Figure 13. Analog Input Equivalent Circuit
Figure 14. Simple RC Antialiasing Circuit
V
V
IN
IN
(+)
(–)
18
16
500
500
Z
CLKIN
IN
=
4
R
R
×
Φ
10
f
A
CLKIN
Φ
Φ
Φ
Φ
9
A
A
B
B
Φ
B
C
C
Φ
A
k
Ω
Φ
GROUND
AD7722
2pF
2pF
B
AC
V
V
IN
IN
(–)
AD7722
(+)
AD7722