7.3.1.1 Decoding Temperature Data

The TMP461-EP temperature registers use a 12-bit format. The 12 bits are aligned to the left side, or most

significant side, of the 16-bit word. The four unused bits are on the right side, or the least significant side.

For this reason, a shift is needed to discard the extra bits. 2’s complement is employed to describe negative

temperatures. C code can easily convert the 2’s complement data when the data is typecast into the correct

signed data type. Q notation describes the number of bits that represent a fractional result. 4 bits of fractional

data, known as Q4, offers 0.0625°C resolution.

Table 7-3. 12-Bit Q4 Encoding Parameters

PARAMETER

VALUE

Bits

12

Q

4

Resolution

0.0625

Range (+)

127.9375

Range (-)

-128

First Byte Integer C

Yes

25˚C

0x1900

Table 7-4. 12-Bit Q4 Bit Values

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

Sign

64

32

16

8

4

2

1

0.5

0.25

0.125 0.0625

-

-

-

-

-128

64

32

16

8

4

2

1

1/2

1/4

1/8

1/16

-

-

-

-

-27

26

25

24

23

22

21

20

2-1

2-2

2-3

2-4

-

-

-

-

/* 12-bit format has 4 bits discarded by right shift

q4 is 0.062500 resolution

the following bytes represent 24.5C */

uint8_t byte1 = 0x18;

uint8_t byte2 = 0x80;

float f = (((int8_t) byte1 << 8 | byte2) >> 4) * 0.0625f;

int mC = (((int8_t) byte1 << 8 | byte2) >> 4) * 1000 >> 4;

int C = (int8_t) byte1;

7.3.2 Series Resistance Cancellation

Series resistance cancellation automatically eliminates the temperature error caused by the resistance of the

routing to the remote transistor or by the resistors of the optional external low-pass filter. A total of up to

1kΩ of series resistance can be canceled by the TMP461-EP device, thus eliminating the need for additional

characterization and temperature offset correction. See Figure 6-3 for details on the effects of series resistance

on sensed remote temperature error.

7.3.3 Differential Input Capacitance

The TMP461-EP device tolerates differential input capacitance of up to 1000pF with minimal change in

temperature error. The effect of capacitance on the sensed remote temperature error is illustrated in Figure

6-6 (Remote Temperature Error vs Differential Capacitance).

7.3.4 Filtering

Remote junction temperature sensors are typically implemented in a noisy environment. Noise is most often

created by fast digital signals that can corrupt measurements. The TMP461-EP device has a built-in, 65kHz

filter on the D+ and D– inputs to minimize the effects of noise. However, a bypass capacitor placed differentially

across the inputs of the remote temperature sensor is recommended to make the application more robust

against unwanted coupled signals. For this capacitor, select a value between 100pF differential and 1nF. Some

TMP461-EP

SNIS241 – SEPTEMBER 2025

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