AD7938 Analog Devices, AD7938 Datasheet - Page 17

• Manufacturer Part Number: AD7938
• Description: 8-Channel, 1.5 MSPS, 12-Bit Parallel ADCs with a Sequencer
• Manufacturer: Analog Devices

Specifications of AD7938

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 1.5MSPS
• Interface: Byte,Par
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (2Vref) p-p,5V p-p,Uni (Vref),Uni (Vref) x 2,Uni 2.5V,Uni 5.0V
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

Data Sheet
SHADOW REGISTER
The shadow register on the AD7938/AD7939 is an 8-bit, write-
only register. Data is loaded from DB0 to DB7 on the rising
edge of WR . The eight LSBs load into the shadow register. The
information is written into the shadow register provided that
the SEQ and SHDW bits in the control register were set to 0 and
1, respectively, in the previous write to the control register. Each
bit represents an analog input from Channel 0 through Channel 7.
A sequence of channels can be selected through which the
AD7938/AD7939 cycles with each consecutive conversion after
the write to the shadow register. To select a sequence of
channels to be converted, if operating in single-ended mode or
Pseudo Mode 2, the associated channel bit in the shadow
register must be set for each required analog input. When
Table 12. Shadow Register Bit Functions
MSB
DB7
V
IN
7
DB6
V
IN
6
DB5
V
IN
5
DB4
V
IN
4
Rev. C | Page 17 of 36
DB3
V
IN
3
operating in fully differential mode or Pseudo Mode 1, the
associated pair of channel bits must be set for each pair of
analog inputs required in the sequence. With each consecutive
CONVST pulse after the sequencer has been set up, the
AD7938/AD7939 progress through the selected channels in
ascending order, beginning with the lowest channel. This
continues until a write operation occurs with the SEQ and
SHDW bits configured in any way except 1, 0 (see Table 11).
When a sequence is set up in fully differential mode or Pseudo
Mode 1, the ADC does not convert on the inverse pairs (that is,
V
outlined in Table 12. See the Analog Input Selection section for
further information on using the sequencer.
IN
1, V
IN
0). The bit functions of the shadow register are
DB2
V
IN
2
DB1
V
IN
1
AD7938/AD7939
LSB
DB0
V
IN
0