AD7938-6 Analog Devices, AD7938-6 Datasheet - Page 17

• 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

Data Sheet
TYPICAL CONNECTION DIAGRAM
Figure 18 shows a typical connection diagram for the
AD7938-6. The AGND and DGND pins are connected together
at the device for good noise suppression. The V
is decoupled to AGND with a 0.47 μF capacitor to avoid noise
pickup if the internal reference is used. Alternatively, V
can be connected to an external reference source. In this case,
the reference pin should be decoupled with a 0.1 μF capacitor.
In both cases, the analog input range can either be 0 V to V
(RANGE bit = 0) or 0 V to 2 × V
analog input configuration can be either eight single-ended
inputs, four differential pairs, four pseudo differential pairs, or
seven pseudo differential inputs (see Table 9). The V
connected to either a 3 V or 5 V supply. The voltage applied to
the V
here, it is connected to the same 3 V supply of the microprocessor
to allow a 3 V logic interface (see the Digital Inputs section).
0 TO 2 × V
0 TO V
V
2.5V
DRIVE
000...001
000...000
100...010
100...001
100...000
011...110
011...111
111...111
REF
with Twos Complement Output Coding and 2 × V
REF
REF
input controls the voltage of the digital interface and
/
Figure 17. AD7938-6 Ideal Transfer Characteristic
V
V
V
AGND
DGND
V
Figure 18. Typical Connection Diagram
DD
IN
IN
REFIN
0
7
0.1µF
–V
REF
/V
1 LSB = 2 × V
REFOUT
AD7938-6
+ 1 LSB
0.1µF EXTERNAL V
0.47µF INTERNAL V
10µF
REF
DB11/DB9
V
CONVST
REF
3V/5V
SUPPLY
REF
/4096
V
CLKIN
BUSY
DRIVE
W/B
DB0
(RANGE bit = 1). The
WR
RD
CS
REF
REF
0.1µF
+V
REF
REFIN
REF
– 1 LSB
Range
/V
10µF
REFIN
DD
SUPPLY
REFOUT
pin is
3V
/V
REFOUT
REF
pin
Rev. C | Page 17 of 32
ANALOG INPUT STRUCTURE
Figure 19 shows the equivalent circuit of the analog input
structure of the AD7938-6 in differential/pseudo differential
mode. In single-ended mode, V
The four diodes provide ESD protection for the analog inputs.
Care must be taken to ensure that the analog input signals never
exceed the supply rails by more than 300 mV. This causes these
diodes to become forward-biased and start conducting into the
substrate. These diodes can conduct up to 10 mA without
causing irreversible damage to the part.
The C1 capacitors in Figure 19 are typically 4 pF and can
primarily be attributed to pin capacitance. The resistors are
lumped components made up of the on resistance of the
switches. The value of these resistors is typically about 100 Ω.
The C2 capacitors are the ADC sampling capacitors and
typically have a capacitance of 45 pF.
For ac applications, removing high frequency components from
the analog input signal is recommended by the use of an RC
low-pass filter on the relevant analog input pins. In applications
where harmonic distortion and signal-to-noise ratio are critical,
the analog input should be driven from a low impedance
source. Large source impedances significantly affect the ac
performance of the ADC. This may necessitate the use of an
input buffer amplifier. The choice of the op amp is a function of
the particular application.
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum
source impedance depends on the amount of THD that can be
tolerated. The THD increases as the source impedance increases
and performance degrades. Figure 20 and Figure 21 show a
graph of the THD vs. source impedance with a 50 kHz input
tone for both V
and differential mode, respectively.
Conversion Phase: Switches Open, Track Phase: Switches Closed
V
V
IN+
Figure 19. Equivalent Analog Input Circuit,
IN–
DD
= 5 V and V
C1
C1
V
V
DD
DD
D
D
D
D
DD
IN−
= 3 V in single-ended mode
is internally tied to AGND.
R1
R1
C2
C2
AD7938-6