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參數(shù)資料
| 型號： | AB-113 |
| 英文描述： | AB-113 - ACCESSING THE ADS1210 DEMO BOARD WITH YOUR PC |
| 中文描述： | 抗體- 113 -快捷地通過個人電腦上的ADS1210演示板 |
| 文件頁數(shù)： | 2/3頁 |
| 文件大?。?/td> | 46K |
| 代理商： | AB-113 |
2
The key differentiating performance characteristics of these
12-bit A/D converters are summarized in Table I. In this
product line, the A/D converter input configuration is either
single-ended or differential. The single-ended style input
stage has a additional “sense” (V
IN–
) pin. The sense pin, in
conjunction with the analog input pin, rejects small signals
(
±
200mV) common to both pins. This allows the user of the
device to extend the signal lines coming to the device over
a longer distance, giving the circuit a better chance to reject
undesired radiated signals. The minimum LSB size is deter-
mined by the number of bits (in all these devices, 12 bits)
and the minimum voltage that is assignable to the LSB,
given the input voltage range. The minimum achievable
LSB size is set by the actual accuracy of the converter with
the given minimum input voltage range. The accuracy of the
converter is defined by the measured rms (one sigma) noise
levels at the output of the device after multiple conversions.
This unique use of V
REF
adjusts the full-scale range as well
as the LSB size externally.
These converters can be used in a wide range of applica-
tions, one being thermocouple interface. A variety of ther-
mocouples can be used for this application, such as “K”, “J”
or “E” type devices. The thermocouple is constructed with
two dissimilar metals that are connected together at one
point. A voltage is created as a result of the temperature
difference from one end of the metals to the other. The
thermocouple used in this particular circuit is a type “E”.
The materials used to build a type “E” thermocouple are
Chromel and Constantan. The sensitivity of four different
thermocouples is shown in the Table II below.
used as a temperature reference point to eliminate the volt-
age errors created by the two thermocouples built with the
E-Type thermocouple leads are both connected to copper
traces. At the isothermal block, a diode is used in conjunc-
tion with the resistors R
1
through R
6
to zero out the undesir-
able effects of the thermocouples that are built into the
circuit as a result of connecting Chromel and Constantan
wires to the PCB traces. The selection of the values of the
resistors R
1
through R
5
depend on the type of thermocouple
used, power requirements of the diode (D
1
), A/D converter
input offsets and overall power consumption requirements.
In this circuit, the current to the CJC circuit is designed to a
nominal 35
μ
A.
A designed-in offset ensures that offset variation from de-
vice to device of the A/D converter does not compromise the
lower temperature readings. The difference at 0
°
C between
the two inputs of the ADS7816 is designed to have a 5mV
difference between the non-inverting and inverting inputs of
the A/D converter. This 5mV difference accounts for tem-
peratures below 0
°
C as well as a possible offset voltage with
the A/D converter of 1mV (max). The ratio of the voltage
divider of R
2
and R
4
is calculated to equal the drift of the “E”
thermocouple (58
μ
V/
°
C) in relation to the diode drift
(–2.1mV/
°
C). The input range of the ADS7816 on the non-
inverting input is equal to V
IN+
minus V
IN–
.
An open circuit indicator is implemented with the inclusion
of R
6
. To insure that R
6
does not interface with the normal
operation of the circuit, a high value is chosen, such as 1M
.
In the event that the lines to the thermocouple are broken, the
inverting input to the A/D converter immediately becomes
the voltage at the diode, approximately 0.6V. This voltage
easily exceeds any voltage that the thermocouple can gener-
ate as a result of high temperature exposure. The digital
conversion of the diode voltage produces a full-scale output,
which is flagged as an error condition by the
μ
Processor
(U
4
).
Typically, a thermocouple output voltage is gained by an
analog front end, such as an instrumentation amplifier. The
analog gain cell is set to insure that the signal full scale
swing is equivalent to the A/D converter input range. The
instrumentation amplifier can be eliminated by using the
A/D converter with a smaller input range. This can be done
by adjusting the reference voltage of the converter.
The ADS7816 is a 12-bit A/D converter with a sampling
input. The input range of the ADS7816 A/D converter is
equal to V
IN+
minus V
IN–
. The sense pin (V
IN–
) input range
is
±
200mV. Reduced V
REF
operates in the application as a
gain increase by reducing the FSR of the converter. The
A/D converter’s input range is decreased, while the con-
verter continues to have 12-bit resolution. The multiplexer
(U
3
) uses the voltages generated by a voltage divider across
the power supply. The range and absolute values of the
voltages at the input of the multiplexer are dependent on the
thermocouple type and temperature range of the application.
TEMPERATURE
COEFFICIENT
(
μ
V/
°
C at 0
°
C)
USEFUL
TEMPERATURE
RANGE (
°
C)
ISA
TYPE
METALS USED
FOR WIRES
E
Chromel/Constantan
58.5
0 to +1000
J
Iron/Constantan
50.2
0 to +1000
K
Chromel/Alumel
39.4
0 to +1300
T
Copper/Constantan
38.0
0 to +600
TABLE II. Temperature Coefficients of Various Thermo-
couple Types.
Thermocouples are low impedance, voltage output devices,
requiring a temperature reference or compensation point. No
voltage or current excitation is needed, which is a plus,
however, their sensitivity is very low. For example, the
K-Type thermocouple has approximately a 40
μ
V/
°
C sensi-
tivity to changes in temperature and will change approxi-
mately 50mV for a temperature range of 0
°
C to 1300
°
C.
An application circuit that takes advantage of the unique
voltage reference capability of the ADS7816 is shown in
Figure 1. In this circuit, the two wires from the thermocouple
are brought to an isothermal block. The isothermal block is
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