AD7934-6 Analog Devices, AD7934-6 Datasheet - Page 16

• Manufacturer Part Number: AD7934-6
• Description: 4-Channel, 625 kSPS, 12-Bit Parallel ADC with a Sequencer
• Manufacturer: Analog Devices

Specifications of AD7934-6

• Resolution (bits): 12bit
• # Chan: 4
• 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: SOP

AD7934-6
TYPICAL CONNECTION DIAGRAM
Figure 18 shows a typical connection diagram for the
AD7934-6. The AGND and DGND pins are connected
together at the device for good noise suppression. The
V
capacitor to avoid noise pickup if the internal reference is
used. Alternatively, V
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
or 0 V to 2 × V
ration is either four single-ended inputs, two differential pairs
or two pseudo differential pairs (see Table 9). The V
connects to either a 3 V or 5 V supply. The voltage applied to
the V
Figure 18, 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
REFIN
0 TO V
DRIVE
V
2.5V
011...111
011...110
000...001
000...000
111...111
100...010
100...001
100...000
/V
REF
with Twos Complement Output Coding and 2 x V
REFOUT
REF
REF
input controls the voltage of the digital interface. In
/
pin is decoupled to AGND with a 0.47 μF
REF
V
V
V
AGND
DGND
V
Figure 18. Typical Connection Diagram
Figure 17. Ideal Transfer Characteristic
DD
IN
IN
REFIN
0
3
0.1µF
–V
(RANGE bit = 1). The analog input configu-
REF
/V
1 LSB = 2 × V
REFIN
+
REFOUT
0.1µF EXTERNAL V
0.47µF INTERNAL V
+ 1 LSB
+
AD7934-6
10µF
/V
REFOUT
+
REF
DB11/DB9
V
CONVST
REF
3V/5V
SUPPLY
/4096
V
CLKIN
can be connected to an
BUSY
DRIVE
DB0
W/B
WR
CS
RD
REF
REF
REF
0.1µF
(RANGE bit = 0)
+V
REF
+
REF
– 1 LSB
+
Range
10µF
DD
SUPPLY
pin
3V
Rev. B | Page 16 of 28
ANALOG INPUT STRUCTURE
Figure 19 shows the equivalent circuit of the analog input
structure of the AD7934-6 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 the
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 sampling capacitors of the ADC 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 can 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
differential mode, respectively.
Conversion Phase—Switches Open, Track Phase—Switches Closed
V
V
Figure 19. Equivalent Analog Input Circuit,
IN+
IN–
DD
= 5 V and 3 V, in single-ended mode and fully
C1
C1
V
V
DD
DD
D
D
D
D
IN−
is internally tied to AGND.
R1
R1
C2
C2