AD7711A Analog Devices, AD7711A Datasheet - Page 15

• Manufacturer Part Number: AD7711A
• Description: CMOS, 24-Bit Sigma-Delta, Signal Conditioning ADC with RTD Current Source
• Manufacturer: Analog Devices

Specifications of AD7711A

• Resolution (bits): 24bit
• # Chan: 2
• Sample Rate: 19.5kSPS
• Interface: Ser
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: Bip (Vref)/(PGA Gain),Uni (Vref)/(PGA Gain)
• Adc Architecture: Sigma-Delta
• Pkg Type: DIP,SOIC

Antialias Considerations
The digital filter does not provide any rejection at integer mul-
tiples of the modulator sample frequency (n ¥ 19.5 kHz, where
n = 1, 2, 3 . . . ). This means that there are frequency bands,
± f
where noise passes unattenuated to the output. However, due to
the AD7711A’s high oversampling ratio, these bands occupy
only a small fraction of the spectrum, and most broadband noise
is filtered. In any case, because of the high oversampling ratio, a
simple, RC, single-pole filter is generally sufficient to attenuate
the signals in these bands on the analog input and thus provide
adequate antialiasing filtering.
If passive components are placed in front of the AD7711A, care
must be taken to ensure that the source impedance is low enough
so as not to introduce gain errors in the system. The dc input
impedance for the AD7711A is over 1 GW. The input appears
as a dynamic load that varies with the clock frequency and with
the selected gain (see Figure 7). The input sample rate, as shown
in Table III, determines the time allowed for the analog input
capacitor, C
longer charge time for this capacitor, which may result in gain
errors being introduced on the analog inputs. Table IV shows the
allowable external resistance/capacitance values such that no
gain error to the 16-bit level is introduced, while Table V shows
the allowable external resistance/capacitance values such that no
gain error to the 20-bit level is introduced. Both inputs of the
differential input channels look into similar input circuitry.
Gain
1
2
4
8–128 17.6 kW 4.8 kW
REV. D
Table IV. Typical External Series Resistance That Will Not
Introduce 16-Bit Gain Error
3 dB
wide (f
0
184 kW
88.6 kW 22.1 kW 13.2 kW 3.6 kW
41.4 kW 10.6 kW 6.3 kW
IN
AIN
3 dB
Figure 7. Analog Input Impedance
, to be charged. External impedances result in a
is cutoff frequency selected by FS0 to FS11)
SWITCHING FREQUENCY DEPENDS
ON
External Capacitance (pF)
50
45.3 kW 27.1 kW 7.3 kW
f
CLKIN
11.5pF TYP
7k
AND SELECTED GAIN
R
INT
TYP
100
2.9 kW
C
INT
V
BIAS
AD7711A
IMPEDANCE
500
1.7 kW
790 W
HIGH
1G
1000
4.1 kW
2.0 kW
970 W
440 W
5000
1.1 kW
560 W
270 W
120 W
–15–
Gain
1
2
4
8–128 13.4 kW 3.6 kW
The numbers in Tables IV and V assume a full-scale change on
the analog input. In any case, the error introduced due to longer
charging times is a gain error that can be removed using the
system calibration capabilities of the AD7711A provided that
the resultant span is within the span limits of the system calibra-
tion techniques for the AD7711A.
ANALOG INPUT FUNCTIONS
Analog Input Ranges
Both analog inputs are differential, programmable gain input
channels that can handle either unipolar or bipolar input signals.
The common-mode range of these inputs is from V
provided that the absolute value of the analog input voltage lies
between V
The dc input leakage current is 10 pA maximum at 25∞C
(± 1 nA over temperature). This results in a dc offset voltage
developed across the source impedance. However, this dc offset
effect can be compensated for by a combination of the differen-
tial input capability of the part and its system calibration mode.
Burnout Current
The AIN1(+) input of the AD7711A contains a 4.5 mA current
source that can be turned on/off via the control register. This
current source can be used in checking that a transducer has not
burned out or gone open circuit before attempting to take mea-
surements on that channel. If the current is turned on and al-
lowed to flow into the transducer and a measurement of the
input voltage on the AIN1 input is taken, it can indicate that
the transducer has burnout or gone open circuit. For normal
operation, this burnout current is turned off by writing a 0 to
the BO bit in the control register.
RTD Excitation Current
The AD7711A also contains a 400 mA constab current source
that is provided at the RTD current pin of the device. This
current can be turned on/off via the control register. Writing a 1
to the I/O bit of the control register enables the excitation current.
The temperature coefficient of the RTD current is typical
20 ppm/∞C. For applications where this coefficient is too large,
the following scheme can be used to remove the drift error. The
conversion result from the AD7711A is ratiometric to the V
voltage. Therefore, if the V
temperature coefficient, the temperature drift of the current
source will be removed. Therefore, the reference voltage for the
part should be generated by placing a low TC resistor (6.25 kW
for 2.5 V reference) in series with the constant current. The
RTD current source can be driven to within 2 V of AV
reference input of the AD7711A is differential so the REF
IN(+) and REF IN(–) of the AD7711A are driven from either
side of the resistor.
Table V. Typical External Series Resistance That Will Not
Introduce 20-Bit Gain Error
0
145 kW
70.5 kW 16.9 kW 10 kW
31.8 kW 8.0 kW
SS
– 30 mV and AV
External Capacitance (pF)
50
34.5 kW 20.4 kW 5.2 kW
REF
100
4.8 kW
2.2 kW
DD
voltage varies with the RTD
+ 30 mV.
500
2.5 kW
1.2 kW
550 W
AD7711A
2.8 kW
1.4 kW
670 W
300 W
1000
SS
to AV
DD
. The
5000
700 W
350 W
170 W
80 W
DD
REF
2