CN0287 Datasheet, PDF (5/9 Pages) Analog Devices – Devices Connected

Circuit Note
Protection Circuits
Transient and overvoltage conditions are possible both during
manufacturing and in the field. To achieve a high level of
protection, additional external protection circuitry is necessary
to compliment the IC’s internal integrated protection circuitry.
The external protection adds additional capacitance, resistance,
and leakage. These effects should be carefully considered to
achieve a high level of accuracy. The additional protection
circuitry is shown in Figure 5.
+5V
OVERVOLTAGE
UP TO 30V
15mA
1.69kΩ
5.6V ZENER DIODE
NZH5V6B
+5V
3mA
ADC
+6V,
−1V
300Ω
+5.3V,
−0.3V
INPUT
TVS
30V, 600W
PTVS30VP1UP
SCHOTTKY DIODES
BAV199LT1G
Figure 5. Transient and Overvoltage Protection Circuit
Leakage currents can have a significant effect on RTD
measurements so should be carefully considered. Leakage currents
can also create some error in thermocouple measurements in
the case where long thermocouple leads have significant resistance.
In this circuit, the PTVS30VP1UP transient voltage suppressor
(TVS) quickly clamps any transient voltages to 30 V with only
1 nA typical leakage current at 25°C. A 30 V TVS was chosen to
allow for a 30 V dc overvoltage. A 1.69 kΩ resistor followed by
low leakage BAV199LT1G Schottky diodes are used to clamp the
voltage to the 5 V power rail during transient and dc overvoltage
events. The 1.69 kΩ resistor limits the current through the
external diodes to about 15 mA during a 30 V dc overvoltage
condition. In order to ensure the power rail is able to sink this
current, a Zener diode is used to clamp the power rail to ensure
it does not exceed the absolute maximum rating of any of the
IC’s connected to the supply. The 5.6 V Zener diode (NZH5V6B) is
selected for this purpose. A 300 Ω resistor limits any further
current that could flow into the AD7193 or the ADG738.
Isolation
The ADuM5401 and the ADuM1280 use ADI iCoupler®
technology provide 2500 V rms isolation voltage between the
measurement side and the controller side of the circuit. The
ADuM5401 also provides the isolated power for measurement
side of the circuit. The isoPower technique used in the ADuM5401
uses high frequency switching elements to transfer power
through a transformer. Special care must be taken with the
printed circuit board (PCB) layout to meet emissions standards.
Refer to AN-0971 Application Note for board layout
recommendations.
CN-0287
Thermocouple Configuration Test Results
The performance of the circuit is highly dependent on the
sensor and the configuration of the AD7193. The Type-K
thermocouple output varies from −10 mV to +60 mV,
corresponding to −200°C to +1350°C. The AD7193 PGA is
configured for G = 32. The voltage swing out of the PGA is
−320 mV to +1.92 V, or 2.24 V p-p. With chop enabled,
50 Hz/60Hz noise reduction enabled, and filter word FS[9:0] =
96, the noise distribution histogram for 1024 samples is shown
in Figure 6.
110
100
90
80
70
60
50
40
30
20
10
0
NUMBER OF OCCURENCES
Figure 6. Noise Distribution Histogram of CN-0287 (VDD = 5 V, VREF =
4.096 V, Differential Input, Bipolar, Input Buffer Enable, Output Data rate =
50 Hz, Gain = 32, Chop Enable, 60 Hz Rejection Enable, Sinc⁴)
The resolution of the AD7193 is 24 bits, or 2²⁴ = 16,777,216
codes. The full dynamic range of the AD7193 is 2 × VREF = 2 ×
4.096 V = 8.192 V. The output voltage of the thermocouple after
the PGA is only 2.24 V p-p and does not occupy all the dynamic
range of the AD7193. Therefore the range of the system is
decreased by a factor of 2.24 V/8.192V.
The noise distribution is about 40 codes peak-to-peak. The
noise-free code resolution over the 2.24 Vp-p range of
measurement is given by:
Noise Free Resolution
=
log ₂



16
,
777 ,
400
216
×
2.24 V
8.192 V



(6)
= 16.8 bits
The full-scale temperature range of the Type-K thermocouple is
−200°C to +1350°C, or 1550°C p-p. The 16.8 bits of noise-free
code resolution therefore corresponds to 0.013°C of noise-free
temperature resolution.
Rev. C | Page 5 of 9