AD7765 datasheet(18/21 Pages) AD

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AD7765

Preliminary Technical Data

Rev. PrC | Page 18 of 21

The sampling switches SS1 and SS3 are driven by ICLK,

whereas, the sampling switches SS2 and SS4 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.

Capacitors CPA, CPB1 and CPB2 represent parasitic

capacitances which include the junction capacitances associated

with the MOS switches.

Table 8 Equivalent Component Values

CS1

CS2

CPA

CPB1/2

13pF

5pF

USING THE AD7765

The following is the recommended sequence for powering up

and using the AD7765.

Apply Power

Start clock oscillator, applying MCLK

Take RESET low for a minimum of 1 MCLK cycle

Wait a minimum of 2 MCLK cycles after RESET has

been released.

In circumstances where multiple parts are being

synchronized, a SYNC pulse must be applied to the

parts, otherwise no SYNC pulse is required.

Conditions for applying the SYNC pulse:

(a) The issue of a SYNC pulse to the part must

not coincide with a write to the part.

(b) Ensure that the SYNC pulse is taken low for

a minimum of 2.5 ICLK cycles.

Data can now be read from the part using the default gain

and over range threshold values. The conversion data read

will not be valid however until the settling time of the filter

has passed. When this has occurred, the DVALID status bit

read will be set indicating that the data is indeed valid.

Values for gain and over range threshold registers can be

written or read at this stage.

BIAS RESISTOR SELECTION

The AD7765 requires a resistor to be connected between the

RBIAS pin and AGND. The value for this resistor is dependant on

the reference voltage being applied to the device. The resistor

value should be selected to give a current of 25µA through the

resistor to ground. For a 4.096V reference voltage, the correct

resistor value is 160kΩ.

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