參數(shù)資料
型號(hào)： AD7711SQ
廠商： ANALOG DEVICES INC
元件分類： ADC
英文描述： LC2MOS Signal Conditioning ADC with RTD Excitation Currents
中文描述： 2-CH 24-BIT DELTA-SIGMA ADC, SERIAL ACCESS, CDIP24
封裝： 0.300 INCH, HERMETIC SEALED, CERDIP-24
文件頁(yè)數(shù)： 15/28頁(yè)
文件大小： 242K
代理商： AD7711SQ
2
AD7711
–15–
REV. F
HIGH
IMPEDANCE
>1G
V
C
11.5pF TYP
V
BIAS
R
INT
7k
V
AIN
SWITCHING FREQUENCY DEPENDS
ON
f
CLKIN
AND SELECTED GAIN
AD7711
Figure 7. Analog Input Impedance
Table IV. Typical External Series Resistance Which Will Not
Introduce 16-Bit Gain Error
External Capacitance (pF)
50
100
45.3 k
27.1 k
7.3 k
88.6 k
22.1 k
13.2 k
3.6 k
41.4 k
10.6 k
6.3 k
17.6 k
4.8 k
Gain
0
184 k
500
1000
4.1 k
2.0 k
970
440
5000
1.1 k
560
270
120
1
2
4
8–128
1.7 k
790
2.9 k
Table V. Typical External Series Resistance Which Will Not
Introduce 20-Bit Gain Error
External Capacitance (pF)
50
100
34.5 k
20.4 k
5.2 k
70.5 k
16.9 k
10 k
31.8 k
8.0 k
13.4 k
3.6 k
Gain
0
145 k
500
1000
2.8 k
1.4 k
670
300
5000
700
350
170
80
1
2
4
8–128
2.5 k
1.2 k
550
4.8 k
2.2 k
The numbers in the above tables assume a full-scale change on
the analog input. In any case, the error introduced due to longer
charging times is a gain error which can be removed using the
system calibration capabilities of the AD7711, provided that the
resultant span is within the span limits of the system calibration
techniques for the AD7711.
ANALOG INPUT FUNCTIONS
Analog Input Ranges
Both analog inputs are programmable gain, input channels
which can handle either unipolar or bipolar input signals. The
AIN1 channel is a differential channel having a common-mode
range from V
SS
to AV
DD
, provided that the absolute value of the
analog input voltage lies between V
SS
–30 mV and AV
DD
+30 mV. The AIN2 input channel is a single-ended input that is
referred to AGND.
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 AD7711 contains a 4.5
μ
A 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
allowed flow into the transducer and a measurement of the
input voltage on the AIN1 input is taken, it can indicate that the
transducer has burned out 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 AD7711 also contains two matched 200
μ
A constant cur-
rent sources which are provided at the RTD1 and RTD2 pins of
the device. These currents can be turned on/off via the control
register. Writing a 1 to the RO bit of the control register enables
these excitation currents.
For four-wire RTD applications, one of these excitation cur-
rents is used to provide the excitation current for the RTD, the
second current source can be left unconnected. For three-wire
RTD configurations, the second on-chip current source can be
used to eliminate errors due to voltage drops across lead resis-
tances. Figures 20 to 22 in the APPLICATIONS section show
some RTD configurations with the AD7711.
The temperature coefficient of the RTD current sources is
typically 20 ppm/
°
C with a typical matching between the tem-
perature coefficients of both current sources of 3 ppm/
°
C. For
applications where the absolute value of the temperature coeffi-
cient is too large, the following schemes can be used to remove
the drift error.
The conversion result from the AD7711 is ratiometric to the
V
REF
voltage. Therefore, if the V
REF
voltage varies with the RTD
temperature coefficient, the temperature drift from the current
source will be removed. For four-wire RTD applications, the
reference voltage can be made ratiometric to RTD current
source by using the second current with a low t.c. resistor to
generate the reference voltage for the part. In this case if a
12.5 k
resistor is used, the 200
μ
A current source generates
+2.5 V across the resistor. This +2.5 V can be applied to the
REF IN(+) input of the AD7711 and with the REF IN(–) input
at ground it will supply a V
REF
of 2.5 V for the part. For three-
wire RTD configurations, the reference voltage for the part is
generated by placing a low t.c. resistor (12.5 k
for 2.5 V refer-
ence) in series with one of the constant current sources. The
RTD current sources can be driven to within 2 V of AV
DD
. The
reference input of the AD7711 is differential so the REF IN(+)
and REF IN(–) of the AD7711 are driven from either side of the
resistor. Both schemes ensure that the reference voltage for the
part tracks the RTD current sources over temperature and,
thereby, removes the temperature drift error.
Bipolar/Unipolar Inputs
The two analog inputs on the AD7711 can accept either unipo-
lar or bipolar input voltage ranges. Bipolar or unipolar options
are chosen by programming the B/U bit of the control register.
This programs both channels for either unipolar or bipolar
operation. Programming the part for either unipolar or bipolar
operation does not change any of the input signal conditioning;
it simply changes the data output coding. The data coding is
binary for unipolar inputs and offset binary for bipolar inputs.
The AIN1 input channel is differential and, as a result, the
voltage to which the unipolar and bipolar signals are referenced
is the voltage on the AIN1(–) input. For example, if AIN1(–) is
+1.25 V and the AD7711 is configured for unipolar operation
with a gain of 1 and a V
REF
of +2.5 V, the input voltage range
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