AMC7812SPAPR Datasheet, PDF (37/92 Pages) Texas Instruments – 12-Bit Analog Monitoring and Control Solution with Multichannel ADC, DACs, and Temperature Sensors

AMC7812
www.ti.com
SBAS513E – JANUARY 2011 – REVISED SEPTEMBER 2013
Remote Sensing Diode
Errors in remote temperature sensor readings are typically the consequence of the ideality factor and current
excitation used by the AMC7812 versus the manufacturer-specified operating current for a given transistor. Some
manufacturers specify a low-level (I_LOW) and high-level (I_HIGH) current for the temperature-sensing substrate
transistors. The AMC7812 uses 6μA for I_LOW and 120μA for I_HIGH. The AMC7812 is designed to work with
discrete transistors, such as the 2N3904 and 2N3906. If an alternative transistor is used, the AMC7812 operates
as specified, as long as the following conditions are met:
1. Base-emitter voltage > 0.25V at 6μA, at the highest sensed temperature.
2. Base-emitter voltage < 0.95V at 120μA, at the lowest sensed temperature.
3. Base resistance < 100Ω.
4. Tight control of V_BE characteristics indicated by small variations in h_FE (that is, 50 to 150).
Ideality Factor
The ideality factor (η) is a measured characteristic of a remote temperature sensor diode as compared to an
ideal diode. The AMC7812 allows for different η-factor values, according to Table 3. The AMC7812 is trimmed for
a power-on reset (POR) value of η = 1.008. If η is different, the η-Factor Correction Register can be used. The
value (N_ADJUST) written in this register must be in twos complement format, as shown in Table 3. This value is
used to adjust the effective η-factor according to Equation 2 and Equation 3.
BINARY
0111 1111
0000 1010
0000 1000
0000 0110
0000 0100
0000 0010
0000 0001
0000 0000
1111 1111
1111 1110
1111 1100
1111 1010
1111 1000
1111 0110
1000 0000
Table 3. η-Factor Range (Single Byte)
N_ADJUST
HEX
7F
0A
08
06
04
02
01
00
FF
FE
FC
FA
F8
F6
80
DECIMAL
127
10
8
6
4
2
1
0
–1
–2
–4
–6
–8
–10
–128
η_EFF
1.747977
1.042759
1.035616
1.028571
1.021622
1.014765
1.011371
1.008
1.004651
1.001325
0.994737
0.988235
0.981818
0.975484
0.706542
h_eff
=
1.008 ´ 300
300 - N_ADJUST
(2)
N_ADJUST = 300 -
300 ´ 1.008
h_eff
Where:
η_EFF is the actual ideality of the transistor being used
N_ADJUST is the corrected ideality being used in the calculation
(3)
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