AD7701AQ Analog Devices, AD7701AQ Datasheet - Page 11

• Manufacturer Part Number: AD7701AQ
• Description: LC2MOS 16-Bit A/D Converter
• Manufacturer: Analog Devices

REV. D
VOLTAGE REFERENCE CONNECTIONS
The voltage applied to the V
range. The specified reference voltage is 2.5 V, but the AD7701
will operate with reference voltages from 1 V to 3 V with little
degradation in performance.
The reference input presents exactly the same dynamic load as
the analog input, but in the case of the reference input, source
resistance and long settling time introduce gain errors rather
than offset errors. Fortunately, most precision references have
sufficiently low output impedance and wide enough bandwidth
to settle to 10 V within 62 clock cycles.
The digital filter of the AD7701 removes noise from the
reference input, just as it does with noise at the analog input,
and the same limitations apply regarding lack of noise rejection
at integer multiples of the sampling frequency. If reference noise
is a problem, some voltage references offer noise reduction
schemes using an external capacitor. Alternatively, a simple RC
filter may be used, as shown in Figure 15.
The same considerations apply to this filter as to a filter at the
analog input. In this case:
where:
and
[R
Figure 14. Typical External Reference Connections
f
V
F
CLKIN
FSE
(C
F
+5V
+5V
is the master clock frequency
is the maximum desired error in volts.
10 pF )]
Figure 15. Filtered Reference Input
AD580
f
CLKIN
13k
R
F
LT1019
REF
ln
pin defines the analog input
C
100pF
100 mV C
F
AV
V
AV
AGND
V
AGND
62
REF
REF
DD
DD
AD7701
AD7701
V
FSE
IN
(C
IN
C
F
)
–11–
GROUNDING AND SUPPLY DECOUPLING
AGND is the ground reference voltage for the AD7701, and is
completely independent of DGND. Any noise riding on the
AGND input with respect to the system analog ground will
cause conversion errors. AGND should therefore be used as the
system ground and also as the ground for the analog input and
the reference voltage.
The analog and digital power supplies to the AD7701 are
independent and separately pinned out, to minimize coupling
between analog and digital sections of the device. The digital
filter will provide rejections of broadband noise on the power
supplies, except at integer multiples of the sampling frequency.
Therefore, the two analog supplies should be decoupled to
AGND using 100 nF ceramic capacitors to provide power
supply noise rejections at these frequencies. The two digital
supplies should similarly be decoupled to DGND.
ACCURACY AND AUTOCALIBRATION
Sigma-delta ADCs, like VFCs and other integrating ADCs, do
not contain any source of nonmonotonicity and inherently offer
no-missing-codes performance. The AD7701 achieves excellent
linearity ( 0.0007%) by the use of high quality, on-chip silicon
dioxide capacitors, which have a very low capacitance/voltage
coefficient.
The AD7701 offers two self-calibration modes using the on-chip
calibration microcontroller and SRAM. Table III is a truth table
for the calibration control inputs SC1 and SC2.
In the self-calibration mode, zero-scale is calibrated against the
AGND pin and full scale is calibrated against the V
remove internal errors.
Note that in the bipolar mode the AD7701 calibrates positive
full scale and midscale (bipolar zero).
In the system-calibration mode, the AD7701 calibrates its zero
and full scale to voltages present on the analog input pin in two
sequential steps. This allows system offsets and/or gain errors to
be nulled out.
A typical system calibration scheme is shown in Figure 16. In
normal operation the analog signal is fed to the AD7701 via an
analog multiplexer. When the system is to be calibrated, A
first switched to the system REF LO via the multiplexer and
CAL is strobed high, with SC1 and SC2 both high. AIN is then
switched to the system REF HI and CAL is strobed, with SC1
low and SC2 high. In this way, the effect of all error sources
Figure 16. Typical Connections for System Calibration
SYSTEM
SYSTEM
REF LO
REF HI
A
IN
A0
ANALOG
MUX
A1
CONDITIONING
SIGNAL
AD7701
SDATA
SCLK
CAL
SC1
SC2
AD7701
REF
COMPUTER
pin, to
MICRO
IN
is