AD7856AR-REEL Analog Devices Inc, AD7856AR-REEL Datasheet - Page 29

• Manufacturer Part Number: AD7856AR-REEL
• Description: IC ADC 14BIT 8CH 5V 24-SOIC
• Manufacturer: Analog Devices Inc

Specifications of AD7856AR-REEL

• Rohs Status: RoHS non-compliant
• Number Of Bits: 14
• Sampling Rate (per Second): 285k
• Data Interface: 8051, QSPI™, Serial, SPI™ µP
• Number Of Converters: 2
• Power Dissipation (max): 89.25mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 105°C
• Mounting Type: Surface Mount
• Package / Case: 24-SOIC (0.300", 7.50mm Width)

REV. A
AD7856 to ADSP-21xx Interface
Figure 39 shows the AD7856 interface to the ADSP-21xx. The
ADSP-21xx is the master and the AD7856 is the slave. The
AD7856 is in Interface Mode 2. For the ADSP-21xx the bits in
the serial port control register should be set up as TFSR = RFSR
= 1 (need a frame sync for every transfer), SLEN = 15 (16-bit
word length), TFSW = RFSW = 1 (alternate framing mode for
transmit and receive operations), INVRFS = INVTFS = 1
(active low RFS and TFS), IRFS = 0, ITFS = 1 (External RFS
and internal TFS), and ISCLK = 1 (internal serial clock). The
CLKIN and CONVST signals can be supplied from the ADSP-
21xx or from an external source. The serial clock from the
ADSP-21xx must be inverted before the SCLK pin of the
AD7856. This SCLK could also be used to drive the CLKIN
input of the AD7856. The BUSY signal indicates when the
conversion is finished and may not be required. The data access
and hold times of the ADSP-21xx and the AD7856 allow for a
serial clock of 6 MHz at 5 V.
MASTER
MASTER
68HC11/L11/16
ADSP-21xx
Figure 38. 68HC11 and 68HC16 Interface
MISO
MOSI
SCK
Figure 39. ADSP-21xx Interface
SCK
IRQ
RFS
TFS
IRQ
SS
DT
DR
SPI
DV
4MHz/6MHz
4MHz/6MHz
DD
OPTIONAL
HC16, QSPI
OPTIONAL
OPTIONAL
OPTIONAL
CONVST
SYNC
SCLK
DOUT
BUSY
DIN
CLKIN
CONVST
CLKIN
SCLK
DOUT
SYNC
BUSY
DIN
AD7856
AD7856
SLAVE
SLAVE
–29–
AD7856 to DSP56000/1/2/L002 Interface
Figure 40 shows the AD7856 to DSP56000/1/2/L002 interface.
Here the DSP5600x is the master and the AD7856 is the slave.
The AD7856 is in Interface Mode 2. The setting of the bits in
the registers of the DSP5600x would be for synchronous opera-
tion (SYN = 1), internal frame sync (SCD2 = 1), gated internal
clock (GCK = 1, SCKD = 1), 16-bit word length (WL1 = 1,
WL0 = 0). Since a gated clock is used here the SCLK cannot be
tied to the CLKIN of the AD7856. The SCLK from the DSP5600x
must be inverted before it is applied to the AD7856. Again the
data access and hold times of the DSP5600x and the AD7856
allows for a SCLK of 6 MHz, V
APPLICATION HINTS
Grounding and Layout
The analog and digital supplies to the AD7856 are independent
and separately pinned out to minimize coupling between the
analog and digital sections of the device. The part has very good
immunity to noise on the power supplies as can be seen by the
PSRR vs. Frequency graph. However, care should still be taken
with regard to grounding and layout.
The printed circuit board that houses the AD7856 should be
designed so that the analog and digital sections are separated
and confined to certain areas of the board. This facilitates the
use of ground planes that can be easily separated. A minimum
etch technique is generally best for ground planes as it gives the
best shielding. Digital and analog ground planes should only be
joined in one place. If the AD7856 is the only device requiring
an AGND to DGND connection, the ground planes should
be connected at the AGND and DGND pins of the AD7856. If
the AD7856 is in a system where multiple devices require AGND
to DGND connections, the connection should still be made at
one point only, a star ground point that should be established
as close as possible to the AD7856.
DSP56000/1/2/L002
MASTER
Figure 40. DSP56000/1/2/L002 Interface
SCK
SRD
SC2
STD
IRQ
4MHz/6MHz
OPTIONAL
OPTIONAL
DD
= 5 V.
CONVST
CLKIN
SCLK
DOUT
SYNC
BUSY
DIN
AD7856
AD7856
SLAVE