Datasheet

AD7764
Rev. A | Page 20 of 32
MODULATOR INPUT STRUCTURE
The AD7764 employs a double-sampling front end, as shown in
Figure 38. For simplicity, only the equivalent input circuitry for
V
IN
+
is shown. The equivalent circuitry for V
IN
is the same.
CS2
CPB2
SS4
SH4
CPA
SS2
SH2
CS1
CPB1
SS3
SH3
SS1
SH1
ANALOG
MODULATOR
V
IN
+
06518-027
Figure 38. Equivalent Input Circuit
The SS1 and SS3 sampling switches are driven by ICLK,
whereas the SS2 and SS4 sampling switches are driven by
ICLK
CS1
.
When ICLK is high, the analog input voltage is connected to
CS1. On the falling edge of ICLK, the SS1 and SS3 switches
open, and the analog input is sampled on CS1. Similarly, when
ICLK is low, the analog input voltage is connected to CS2. On
the rising edge of ICLK, the SS2 and SS4 switches open, and the
analog input is sampled on CS2.
The CPA, CPB1, and CPB2 capacitors represent parasitic
capacitances that include the junction capacitances associated
with the MOS switches.
Table 8. Equivalent Component Values
CS2 CPA CPB1/CPB2
13 pF 13 pF 13 pF 5 pF
DRIVING THE MODULATOR INPUTS DIRECTLY
The AD7765 can be configured so that the on-board differential
amplifier can be disabled and the modulator can be driven
directly using discrete amplifiers. This allows the user to lower
the power dissipation.
To power down the on board differential amplifier, the user
issues a write to set the AMP OFF bit in the control register to
logic high (see Figure 39).
SCO (O)
CONTROL REGISTER
ADDRESS 0x0001
AMP OFF MODE
DATA 0x0001
FSI (I)
SDI (I)
32 ×
t
SCO
06518-301
Figure 39. Writing to the AD7764 Control Register Turning Off the On-Board
Differential Amplifier
The AD7764 modulator inputs must have a common-mode
voltage of 2.048 V and adhere to the amplitudes as described in
the AD7764 Input Structure section.
An example of a typical circuit to drive the AD7764 for applica-
tions requiring excellent ac and dc performance is shown in
Figure 40. Either the AD8606 or AD8656 can be used to drive
the AD7764 modulator inputs directly.
Best practice is to short the differential amplifier inputs to
ground through the typical input resistors and leave the typical
feedback resistors in place.
06518-302
1
V
IN
A–
2
V
OUT
A+
R
FB
R
IN
3
V
IN
A+
4
V
OUT
A–
R
FB
R
IN
6
V
IN
+
AD7764
5
V
IN
U1
10k
4.99k
51
0
10k
C2
2
2.048V
U2
10k
4.99k
51
0
10k
C1
2
1.024V
AD8606
AD8655
AD8606
AD8655
ANALOG
INPUT
1
1
–0.5dBFS INPUT SIGNAL AS DESCRIBED IN INPUT STRUCTURE SECTION.
2
SET C1 AND C2 AS REQUIRED FOR APPLICATION INPUT BW AND
ANTI-ALIAS REQUIREMENT.
Figure 40. Driving the AD7764 Modulator Inputs Directly from a Single-
Ended Source (On-Board Differential Amplifier Powered Down)