AD7938 View Datasheet(PDF) - Analog Devices

Part Name

Description

MFG CO.

AD7938

8-Channel, 1.5 MSPS, 12-Bit and 10-Bit Parallel ADCs with a Sequencer

Analog Devices 

AD7938/AD7939

Pin No Mnemonic

20

CS

21

AGND

22

VREFIN/VREFOUT

23 to 30 VIN0 to VIN7

31

VDD

32

W/B

Function

Chip Select. Active low logic input used in conjunction with RD and WR to read conversion data or to write data

to the internal registers.

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.

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 VDD; however, care must be

taken to ensure that the analog input range does not exceed VDD + 0.3 V. See the Reference Section.

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 input range for all input channels can either be 0 V to VREF or 0 V to 2 × VREF, and

the coding can be binary or twos 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.

Power Supply Input. The VDD range for the AD7938/AD7939 is 2.7 V to 5.25 V. The supply should be decoupled

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

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. Unused data

lines when operating in byte transfer mode should be tied off to DGND.

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