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| 型号： | AD7711AQ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 7/28页 |
| 文件大小： | 0K |
| 描述： | IC ADC 24BIT RTD I SOURCE 24CDIP |
| 标准包装： | 15 |
| 位数： | 24 |
| 采样率（每秒）： | 1.03k |
| 数据接口： | 串行 |
| 转换器数目： | 1 |
| 功率耗散（最大）： | 52.5mW |
| 电压电源： | 模拟和数字，双 ± |
| 工作温度： | -40°C ~ 80°C |
| 安装类型： | 通孔 |
| 封装/外壳： | 24-CDIP（0.300"，7.62mm） |
| 供应商设备封装： | 24-CDIP |
| 包装： | 管件 |
| 输入数目和类型： | 1 个单端，单极；1 个单端，双极；1 个差分，单极；1 个差分，双极 |
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2
REV.G
AD7711
–15–
Table IV. External Series Resistance That Will Not Introduce
16-Bit Gain Error
External Capacitance (pF)
Gain
0
50
100
500
1000
5000
1
184 k
W 45.3 kW 27.1 kW 7.3 kW 4.1 kW 1.1 kW
2
88.6 k
W 22.1 kW 13.2 kW 3.6 kW 2.0 kW 560 W
4
41.4 k
W 10.6 kW 6.3 kW 1.7 kW 970 W 270 W
8–128
17.6 k
W 4.8 kW 2.9 kW 790 W 440 W 120
W
Table V. External Series Resistance That Will Not Introduce
20-Bit Gain Error
External Capacitance (pF)
Gain
0
50
100
500
1000
5000
1
145 k
W 34.5 kW 20.4 kW 5.2 kW 2.8 kW 700 W
2
70.5 k
W 16.9 kW 10 kW
2.5 k
W 1.4 kW 350 W
4
31.8 k
W 8.0 kW 4.8 kW 1.2 kW 670 W 170 W
8–128
13.4 k
W 3.6 kW 2.2 kW 550 W 300 W 80 W
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 AD7711, provided
the resultant span is within the limits of the system calibration
techniques.
ANALOG INPUT FUNCTIONS
Analog Input Ranges
Both analog inputs are programmable gain input channels that
can handle either unipolar or bipolar input signals. The AIN1
channel is a differential channel with a common-mode range
from VSS to AVDD, provided the absolute value of the analog
input voltage lies between VSS – 30 mV and AVDD + 30 mV.
The AIN2 input channel is a single-ended input that is referred
to as 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 differential input
capability of the part and its system calibration mode.
Burnout Current
The AIN1(+) input of the AD7711 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
allowed 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 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
mA constant cur-
rent sources that 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 4-wire RTD applications, one of these excitation currents is
used to provide the excitation current for the RTD; the second
current source can be left unconnected. For 3-wire RTD con-
figurations, the second on-chip current source can be used to
eliminate errors due to voltage drops across lead resistances.
Figures 19 to 21 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
VREF voltage. Therefore, if the VREF voltage varies with the RTD
temperature coefficient, the temperature drift from the current
source will be removed. For 4-wire RTD applications, the refer-
ence voltage can be made ratiometric to RTD current source
by using the second current with a low TC resistor to generate
the reference voltage for the part. In this case, if a 12.5 k
W
resistor is used, the 200
mA 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 VREF of 2.5 V for the part. For 3-wire RTD configura-
tions, the reference voltage for the part is generated by placing a
low TC resistor (12.5 k
W for 2.5 V reference) in series with one
of the constant current sources. The RTD current sources can
be driven to within 2 V of AVDD. 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, remove 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 type of operation
does not change any of the input signal conditioning; it simply
changes the data output coding, using 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 VREF of 2.5 V, the input voltage range on
the AIN1(+) input is 1.25 V to 3.75 V. If AIN1(–) is 1.25 V,
and the AD7711 is configured for bipolar mode with a gain of
1 and a VREF of 2.5 V, the analog input range on the AIN1(+)
input is –1.25 V to +3.75 V. For the AIN2 input, the input
signals are referenced to AGND.
REFERENCE INPUT/OUTPUT
The AD7711 contains a temperature compensated 2.5 V reference
that has an initial tolerance of
±1%. This reference voltage is
provided at the REF OUT pin, and it can be used as the reference
voltage for the part by connecting the REF OUT pin to the REF
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