AD7765 Analog Devices, AD7765 Datasheet - Page 19

• Manufacturer Part Number: AD7765
• Description: 24-Bit, 156 kSPS, 112 dB Sigma-Delta ADC with On-Chip Buffers and Serial Interface
• Manufacturer: Analog Devices

Specifications of AD7765

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: 40MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: 6.5 V p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: SOP

MODULATOR INPUT STRUCTURE
The AD7765 employs a double-sampling front end, as shown in
Figure 32. For simplicity, only the equivalent input circuitry for
V
The SS1 and SS3 sampling switches are driven by ICLK,
whereas the SS2 and SS4 sampling switches are driven by ICLK .
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 capa-
citances that include the junction capacitances associated with
the MOS switches.
Table 8. Equivalent Component Values
CS1
13 pF
IN
+ is shown. The equivalent circuitry for V
V
IN
+
CPA
CS2
13 pF
Figure 32. Equivalent Input Circuit
SH1
SH2
SS1
SS2
CPB1
CPB2
CS1
CS2
SS3
SS4
CPA
13 pF
SH3
SH4
IN
MODULATOR
− is the same.
ANALOG
CPB1/CPB2
5 pF
Rev. A | Page 19 of 32
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 33).
SCO (O)
The AD7765 modulator inputs must have a common-mode
voltage of 2.048 V and adhere to the amplitudes as described in
the AD7765 Input Structure section.
An example of a typical circuit to drive the AD7765 for
applications requiring excellent ac and dc performance is shown
in Figure 34. Either the
drive the AD7765 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.
Figure 33. Writing to the AD7765 Control Register Turning Off the On-Board
SDI (I)
FSI (I)
1
2
ANALOG
Figure 34. Driving the AD7765 Modulator Inputs Directly from a Single-
INPUT
–0.5dBFS INPUT SIGNAL AS DESCRIBED IN INPUT STRUCTURE SECTION.
SET C1 AND C2 AS REQUIRED FOR APPLICATION INPUT BW AND
ANTI-ALIAS REQUIREMENT.
Ended Source (On-Board Differential Amplifier Powered Down)
1
1.024V
10kΩ
4.99kΩ
CONTROL REGISTER
ADDRESS 0x0001
10kΩ
AD8606
AD8655
C1
U2
2
Differential Amplifier
AD8606
2.048V
51Ω
0Ω
10kΩ
4.99kΩ
32 ×
t
or
SCO
6
V
IN
10kΩ
V
AD8656
AD8606
AD8655
C2
U1
1
A–
IN
R
+
2
R
IN
FB
V
AMP OFF MODE
OUT
AD7764/
AD7765
DATA 0x0001
2
51Ω
0Ω
A+
can be used to
V
V
IN
5
IN
–
3
A+
R
R
IN
FB
V
AD7765
OUT
4
A–