AD677
FUNCTIONAL DESCRIPTION
The AD677 is a multipurpose 16-bit analog-to-digital converter
and includes circuitry which performs an input sample/hold
function, ground sense, and autocalibration. These functions
are segmented onto two monolithic chips—an analog signal pro-
cessor and a digital controller. Both chips are contained within
the AD677 package.
The AD677 employs a successive-approximation technique to
determine the value of the analog input voltage. However, in-
stead of the traditional laser-trimmed resistor-ladder approach,
this device uses a capacitor-array, charge redistribution tech-
nique. Binary-weighted capacitors subdivide the input sample to
perform the actual analog-to-digital conversion. The capacitor
array eliminates variation in the linearity of the device due to
temperature-induced mismatches of resistor values. Since a
capacitor array is used to perform the data conversions, the
sample/hold function is included without the need for additional
external circuitry.
Initial errors in capacitor matching are eliminated by an
autocalibration circuit within the AD677. This circuit employs
an on-chip microcontroller and a calibration DAC to measure
and compensate capacitor mismatch errors. As each error is
determined, its value is stored in on-chip memory (RAM).
Subsequent conversions use these RAM values to improve con-
version accuracy. The autocalibration routine may be invoked
at any time. Autocalibration insures high performance while
eliminating the need for any user adjustments and is described
in detail below.
The microcontroller controls all of the various functions within
the AD677. These include the actual successive approximation
algorithm, the autocalibration routine, the sample/hold opera-
tion, and the internal output data latch.
AUTO CALIBRATION
In most applications, it is sufficient to calibrate the AD677 only
upon power-up, in which case care should be taken that the
power supplies and voltage reference have stabilized first. If
calibration is not performed, the AD677 may come up in an un-
known state, or performance could degrade to as low as 10 bits.
CONVERSION CONTROL
The AD677 is controlled by two signals: SAMPLE and CLK,
as shown in Figure 2. It is assumed that the part has been cali-
brated and the digital I/O pins have the levels shown at the start
of the timing diagram.
A conversion consists of an input acquisition followed by 17
clock pulses which execute the 16-bit internal successive ap-
proximation routine. The analog input is acquired by taking the
SAMPLE line HIGH for a minimum sampling time of t
S
. The
actual sample taken is the voltage present on V
IN
one aperture
delay after the SAMPLE line is brought LOW, assuming the
previous conversion has completed (signified by BUSY going
LOW). Care should be taken to ensure that this negative edge is
well defined and jitter free in ac applications to reduce the un-
certainty (noise) in signal acquisition. With SAMPLE going
LOW, the AD677 commits itself to the conversion—the input
at V
IN
is disconnected from the internal capacitor array, BUSY
goes HIGH, and the SAMPLE input will be ignored until the
conversion is completed (when BUSY goes LOW). SAMPLE
must be held LOW for a minimum period of time t
SL
. A period
of time t
FCD
after bringing SAMPLE LOW, the 17 CLK cycles
are applied; CLK pulses that start before this period of time are
ignored. BUSY goes HIGH t
SB
after SAMPLE goes LOW, sig-
nifying that a conversion is in process, and remains HIGH until
the conversion is completed. As indicated in Figure 2, the twos
complement output data is presented MSB first. This data may
be captured with the rising edge of SCLK or the falling edge of
CLK, beginning with pulse #2. The AD677 will ignore CLK
after BUSY has gone LOW and SDATA or SCLK will not
change until a new sample is acquired.
CONTINUOUS CONVERSION
The AD677 achieves rated performance without the need for
user trims or adjustments. This is accomplished through the use
of on-chip autocalibration.
In the autocalibration sequence, sample/hold offset is nulled by
internally connecting the input circuit to the ground sense cir-
cuit. The resulting offset voltage is measured and stored in
RAM for later use. Next, the capacitor representing the most
significant bit (MSB) is charged to the reference voltage. This
charge is then transferred to a capacitor of equal size (composed
of the sum of the remaining lower weight bits). The voltage that
results represents the amount of capacitor mismatch. A calibra-
tion digital-to-analog converter (DAC) adds an appropriate
value of error correction voltage to cancel this mismatch. This
correction factor is also stored in RAM. This process is repeated
for each of the eight remaining capacitors representing the top
nine bits. The accumulated values in RAM are then used during
subsequent conversions to adjust conversion results accordingly.
As shown in Figure 1, when CAL is taken HIGH the AD677
internal circuitry is reset, the BUSY pin is driven HIGH, and
the ADC prepares for calibration. This is an asynchronous hard-
ware reset and will interrupt any conversion or calibration cur-
rently in progress. Actual calibration begins when CAL is taken
LOW and completes in 85,532 clock cycles, indicated by BUSY
going LOW. During calibration, it is preferable for SAMPLE to
be held LOW. If SAMPLE is HIGH, diagnostic data will appear
on SDATA. This data is of no value to the user.
For maximum throughput rate, the AD677 can be operated in a
continuous convert mode. This is accomplished by utilizing the
fact that SAMPLE will no longer be ignored after BUSY goes
LOW, so an acquisition may be initiated even during the HIGH
time of the 17th CLK pulse for maximum throughput rate
while enabling full settling of the sample/hold circuitry. If
SAMPLE is already HIGH during the rising edge of the 17th
CLK, then an acquisition is immediately initiated approxi-
mately 100 ns after the rising edge of the 17th clock pulse.
Care must be taken to adhere to the minimum/maximum tim-
ing requirements in order to preserve conversion accuracy.
GENERAL CONVERSION GUIDELINES
During signal acquisition and conversion, care should be taken
with the logic inputs to avoid digital feedthrough noise. It is
possible to run CLK continuously, even during the sample
period. However, CLK edges during the sampling period, and
especially when SAMPLE goes LOW, may inject noise into the
sampling process. The AD677 is tested with no CLK cycles
during the sampling period. The BUSY signal can be used to
prevent the clock from running during acquisition, as illustrated
–8–
REV. A
AD [ ANALOG DEVICES ]
