EVAL-AD7656CB View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-AD7656CB' PDF : 25 Pages View PDF
AD7656/AD7657/AD7658
A N A L OG SUPPLY
VOLTAGE 5V1 +
10µF
100nF
DVCC
+
100nF 10µF
DIGITAL SUPPLY
VOLTAGE +3V OR +5V
+
100nF 10µF
+9.5V TO +16.5V2
SUPPLY
+
10µF
100nF
AGND AVCC DVCC DGND
VDD
AGND
VDRIVE DGND
D0 TO D15
+
10µF
2.5V
REF
+
10µF
100nF
100nF
REFCAPA, B, C
CONVST A, B, C
CS
AGND
RD
AD7656/AD7657/AD7658 BUSY
REFIN/OUT
RESET
AGND
SER/PAR
H/S
SIX ANALOG
INPUTS
W/B
RANGE
–9.5V TO –16.5V2
SUPPLY
10µF
+
100nF
VS S
AGND
STBY
PARALLEL
INTERFACE
µP/µC/DSP
VDRIVE
1DECOUPLING SHOWN ON THE AVCC PIN APPLIES TO EACH AVCC PIN.
2SEE POWER SUPPLY CONFIGURATION SECTION.
Figure 26. Typical Connection Diagram
Data Sheet
The VDRIVE supply is connected to the same supply as the
processor. The voltage on VDRIVE controls the voltage value
of the output logic signals.
The VDD and VSS signals should be decoupled with a minimum
10 µF decoupling capacitor. These supplies are used for the high
voltage analog input structures on the AD7656/AD7657/AD7658
analog inputs.
DRIVING THE ANALOG INPUTS
Together, the driver amplifier and the analog input circuit used
for the AD7656 must settle for a full-scale step input to a 16-bit
level (0.0015%), which is within the specified 550 ns acquisition
time of the AD7656. The noise generated by the driver amplifier
needs to be kept as low as possible to preserve the SNR and
transition noise performance of the AD7656.
The driver also needs to have a THD performance suitable to
that of the AD7656. The AD8021 meets all these requirements.
The AD8021 needs an external compensation capacitor of
10 pF. If a dual version of the AD8021 is required, the AD8022
can be used. The AD8610 and the AD797 can also be used to
drive the AD7656/AD7657/AD7658.
INTERFACE SECTION
The AD7656/AD7657/AD7658 provide two interface options, a
parallel interface and a high speed serial interface. The required
interface mode is selected via the SER/PAR pin. The parallel
interface can operate in word (W/B = 0) or byte (W/B = 1) mode.
The interface modes are discussed in the following sections.
Parallel Interface (SER/PAR = 0)
The AD7656/AD7657/AD7658 consist of six 16-/14-/12-bit
ADCs, respectively. A simultaneous sample of all six ADCs can
be performed by connecting all three CONVST pins together,
CONVST A, CONVST B, and CONVST C. The AD7656/AD7657/
AD7658 need to see a CONVST pulse to initiate a conversion;
this should consist of a falling CONVST edge followed by a
rising CONVST edge. The rising edge of CONVSTx initiates
simultaneous conversions on the selected ADCs. The AD7656/
AD7657/AD7658 contain an on-chip oscillator that is used to
perform the conversions. The conversion time, tCONV, is 3 µs.
The BUSY signal goes low to indicate the end of conversion.
The falling edge of the BUSY signal is used to place the track-
and-hold into track mode. The AD7656/AD7657/AD7658 also
allow the six ADCs to be converted simultaneously in pairs by
pulsing the three CONVST pins independently. CONVST A is
used to initiate simultaneous conversions on V1 and V2,
CONVST B is used to initiate simultaneous conversions on
V3 and V4, and CONVST C is used to initiate simultaneous
conversions on V5 and V6. The conversion results from the
simultaneously sampled ADCs are stored in the output data
registers.
Data can be read from the AD7656/AD7657/AD7658 via the
parallel data bus with standard CS and RD signals (W/B = 0).
To read the data over the parallel bus, SER/PAR should be tied
low. The CS and RD input signals are internally gated to enable
the conversion result onto the data bus. The data lines DB0 to
DB15 leave their high impedance state when both CS and RD
are logic low.
Rev. D | Page 22 of 32
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