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a
LC
2
MOS, High Speed
1-, 4- & 8-Channel 10-Bit ADCs
AD7776/AD7777/AD7778*
FUNCTIONAL BLOCK DIAGRAMS
CS RD WR BUSY/INT
AGND
DB0–DB9
AD7776
10
DGND
CLKIN
10
CONTROL
REGISTER
ADCREG1
T/H
REFOUT
REFIN
REF
AGND
C
REFIN
RTN
V
SWING

V
BIAS
A
IN
1 MUX
REFIN
10-BIT
ADC
CONTROL LOGIC
V
CC
AGND
DB0–DB9
AD7777
10
CONTROL
REGISTER
DGND
CLKIN
10
ADCREG2
ADCREG1
T/H
1
REFOUT
REFIN
REF
AGND
C
REFIN
V
SWING

V
BIAS
A
IN
1
A
IN
2
A
IN
3
A
IN
4
MUX
1
REFIN
T/H
2
MUX
2
10-BIT
ADC
CONTROL LOGIC
V
CC
RTN
AGND
DB0–DB9
AD7778
10
CONTROL
REGISTER
DGND
CLKIN
CS RD WR BUSY/INT
10
ADCREG2
ADCREG1
T/H
1
REFOUT
REFIN
REF
AGND
C
REFIN
V
SWING

V
BIAS
A
IN
1
A
IN
2
A
IN
3
A
IN
4
A
IN
5
A
IN
6
A
IN
7
A
IN
8
MUX
1
REFIN
T/H
2
MUX
2
10-BIT
ADC
CONTROL LOGIC
V
CC
RTN
FEATURES
AD7776: Single Channel
AD7777: 4-Channel
AD7778: 8-Channel
Fast 10-Bit ADC: 2.5 ms Worst Case
+5 V Only
Half-Scale Conversion Option
Fast Interface Port
Power-Down Mode
APPLICATIONS
HDD Servos
Instrumentation
GENERAL DESCRIPTION
The AD7776, AD7777 and AD7778 are a family of high speed,
multichannel, 10-bit ADCs primarily intended for use in R/W
head positioning servos found in high density hard disk drives.
They have unique input signal conditioning features that make
them ideal for use in such single supply applications.
By setting a bit in a control register within both the four-channel
version, AD7777, and the eight-channel version, AD7778, the
input channels can either be independently sampled or any two
channels of choice can be simultaneously sampled. For all ver-
sions the specified input signal range is of the form V
BIAS
±
V
SWING
. However, if the RTN pin is biased at, say, 2 V the
analog input signal range becomes 0 V to +2 V for all input
channels. This is covered in more detail under the section
Changing the Analog Input Voltage Range. The voltage V
BIAS
is the offset of the ADC’s midpoint code from ground and is
supplied either by an onboard reference available to the user
(REFOUT) or by an external voltage reference applied to
REFIN. The full-scale range (FSR) of the ADC is equal to
2 V
SWING
where V
SWING
is nominally equal to REFIN/2. Addi-
tionally, when placed in the half-scale conversion mode, the
value of REFIN is converted. This allows the channel offset(s)
to be measured.
Control register loading and ADC register reading, channel se-
lect and conversion start are under the control of the µP. The
twos complemented coded ADCs are easily interfaced to a stan-
dard 16-bit MPU bus via their 10-bit data port and standard
microprocessor control lines.
The AD7776/AD7777/AD7778 are fabricated in linear compat-
ible CMOS (LC
2
MOS), an advanced, mixed technology process
that combines precision bipolar circuits with low power CMOS
logic. The AD7776 is available in a 24-pin SOIC package; the
AD7777 is available in both 28-pin DIP and 28-pin SOIC pack-
ages; the AD7778 is available in a 44-pin PQFP package.
*Protected by U.S. Patent No. 4,990,916.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700 World Wide Web Site: http://www.analog.com
Fax: 617/326-8703 © Analog Devices, Inc., 1997