AD7938-6 Analog Devices, AD7938-6 Datasheet - Page 20

• Manufacturer Part Number: AD7938-6
• Description: 8-Channel, 625 kSPS, 12-Bit Parallel ADCs with a Sequencer
• Manufacturer: Analog Devices

Specifications of AD7938-6

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 625kSPS
• Interface: Par
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: Uni (Vref),Uni (Vref) x 2
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

AD7938-6
Another method of driving the AD7938-6 is to use the
(or equivalent) differential amplifier. The AD8138 can be used
as a single-ended-to-differential amplifier or as a differential-to-
differential amplifier. The device is as easy to use as an op amp
and greatly simplifies differential signal amplification and driving.
Pseudo Differential Mode
The AD7938-6 can have four pseudo differential pairs (Pseudo
Mode 1) or seven pseudo differential inputs (Pseudo Mode 2)
by setting the MODE0 and MODE1 bits in the control register
to 1, 0 and 1, 1, respectively. In the case of the four pseudo
differential pairs, V
must have an amplitude of V
range chosen) to make use of the full dynamic range of the part.
A dc input is applied to the V
input provides an offset from ground or a pseudo ground for
the V
inputs, the seven analog input signals inputs are referred to a dc
voltage applied to V
The benefit of pseudo differential inputs is that they separate
the analog input signal ground from the ADC ground allowing
dc common-mode voltages to be cancelled. The specified
voltage range for the V
GND
V
GND
REF
2 × V
IN+
V
REF
Figure 27. Dual Op Amp Circuit to Convert a Single-Ended
Figure 28. Dual Op Amp Circuit to Convert a Single-Ended
input. In the case of the seven pseudo differential
REF
p-p
p-p
Bipolar Signal into a Differential Unipolar Signal
220Ω
440Ω
20kΩ
440Ω
20kΩ
Unipolar Signal into a Differential Signal
A
A
IN
IN
+ is connected to the signal source, which
220Ω
220Ω
220Ω
220Ω
220Ω
220Ω
7.
IN−
10kΩ
10kΩ
V+
V–
V+
V–
V+
V–
V+
V–
pin while in pseudo differential mode
REF
IN−
(or 2 × V
27Ω
27Ω
27Ω
27Ω
pin. The voltage applied to this
REF
depending on the
3.75V
2.5V
1.25V
3.75V
2.5V
1.25V
3.75V
2.5V
1.25V
3.75V
2.5V
1.25V
V
V
AD7938-6
V
V
AD7938-6
IN+
IN–
IN+
IN–
0.47µF
0.47µF
AD8138
V
V
REF
REF
Rev. C | Page 20 of 32
is −0.1 V to +0.4 V; however, typically this range can extend to
−0.3 V to +0.7 V when V
V
differential mode.
ANALOG INPUT SELECTION
As shown in Table 9, the user can set up their analog input
configuration by setting the values in the MODE0 and MODE1
bits in the control register. Assuming the configuration has been
chosen, there are different ways of selecting the analog input to
be converted depending on the state of the SEQ and SHDW bits
in the control register.
Traditional Multichannel Operation (SEQ = 0, SHDW = 0)
Any one of eight analog input channels or four pairs of channels
can be selected for conversion in any order by setting the SEQ
and SHDW bits in the control register to 0. The channel to be
converted is selected by writing to the address bits, ADD2 to
ADD0, in the control register to program the multiplexer prior
to the conversion. This mode of operation is that of a traditional
multichannel ADC where each data write selects the next
channel for conversion. Figure 30 shows a flow chart of this
mode of operation. The channel configurations are shown in
Table 9.
DD
= 5 V. Figure 29 shows a connection diagram for pseudo
*ADDITIONAL PINS OMITTED FOR CLARITY.
V
REF
Figure 30. Traditional Multichannel Operation Flow Chart
Figure 29. Pseudo Differential Mode Connection Diagram
ISSUE CONVST PULSE TO INITIATE A CONVERSION
CHANGING THE VALUES OF BITS ADD2 TO ADD0
p-p
INITIATE A WRITE CYCLE TO SELECT THE NEXT
IN THE CONTROL REGISTER. SEQ = SHDW = 0.
SET SEQ = SHDW = 0. SELECT THE DESIRED
INITIATE A READ CYCLE TO READ THE DATA
SET UP OPERATING MODE, ANALOG INPUT
CHANNEL TO CONVERT (ADD2 TO ADD0).
WRITE TO THE CONTROL REGISTER TO
FROM THE SELECTED CHANNEL.
CHANNEL TO BE CONVERTED BY
AND OUTPUT CONFIGURATION
ON THE SELECTED CHANNEL.
POWER ON
DD
DC INPUT
VOLTAGE
= 3 V, or −0.3 V to +1.8 V when
V
V
V
REF
IN+
IN–
AD7938-6*
Data Sheet
0.47µF