AD7765 datasheet(20/33 Pages) AD

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AD7765

Rev. A | Page 19 of 32

MODULATOR INPUT STRUCTURE

The AD7765 employs a double-sampling front end, as shown in

Figure 32. For simplicity, only the equivalent input circuitry for

VIN+ is shown. The equivalent circuitry for VIN− is the same.

CS2

CPB2

SS4

SH4

CPA

SS2

SH2

CS1

CPB1

SS3

SH3

SS1

SH1

ANALOG

MODULATOR

VIN+

Figure 32. Equivalent Input Circuit

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

CS2

CPA

CPB1/CPB2

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 33).

SCO (O)

CONTROL REGISTER

ADDRESS 0x0001

AMP OFF MODE

DATA 0x0001

FSI (I)

SDI (I)

32 ×

tSCO

Figure 33. Writing to the AD7765 Control Register Turning Off the On-Board

Differential Amplifier

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 AD8606 or AD8656 can be used to

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.

VINA–

VOUTA+

RFB

RIN

VINA+

VOUTA–

RFB

RIN

VIN+

AD7764/

AD7765

VIN–

10kΩ

4.99kΩ

51Ω

0Ω

10kΩ

C22

2.048V

10kΩ

4.99kΩ

51Ω

0Ω

10kΩ

C12

1.024V

AD8606

AD8655

AD8606

AD8655

ANALOG

INPUT1

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 34. Driving the AD7765 Modulator Inputs Directly from a Single-

Ended Source (On-Board Differential Amplifier Powered Down)

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