AD7938/AD7939

Preliminary Technical Data

Pin

No

Mnemonic

Function

the internal registers.

21

AGND

Analog Ground. This is the ground reference point for all analog circuitry on the AD7938/AD7939. All analog input

signals and any external reference signal should be referred to this AGND voltage. The AGND and DGND voltages

should ideally be at the same potential and must not be more than 0.3 V apart, even on a transient basis.

22

Reference Input/Output. This pin is connected to the internal reference and is the reference source for the ADC. The

nominal internal reference voltage is 2.5 V and this appears at this pin. This pin can be overdriven by an external

reference. The input voltage range for the external reference is 0.1 V to 3.5 V for the differential mode and is 0.1 V to

23

to

30

Analog Input 0 to Analog Input 7. Eight analog input channels that are multiplexed into the on-chip track-and-hold.

The analog inputs can be programmed to be eight single-ended inputs, four fully differential pairs, four pseudo-

differential pairs, or seven pseudo-differential inputs by setting the MODE bits in the control register appropriately

(see Table 9). The analog input channel to be converted can either be selected by writing to the address bits (ADD2

to ADD0) in the control register prior to the conversion or the on-chip sequencer can be used. The SEQ and SHDW

bits in conjunction with the address bits in the control register allow the shadow register to be programmed. The

complement, depending on the states of the RANGE and CODING bits in the control register. Any unused input

channels should be connected to AGND to avoid noise pickup.

31

AGND with a 0.1 µF capacitor and a 10 µF tantalum capacitor.

32

W/B

Word/Byte Input. When this input is logic high, data is transferred to and from the AD7938/AD7939 in 12-bit/10-bit

words on Pins DB0/DB2 to DB11. When this pin is logic low, byte transfer mode is enabled. Data and the channel ID

are transferred on Pins DB0 to DB7, and Pin DB8/HBEN assumes its HBEN functionality.

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