TMP121 | GUILCOR SENSORS

TMP121
Temperature sensors

High-precision digital temperature sensor with SPI interface, ideal for applications requiring stable and repeatable readings.

Maximum precision
+/- 1,0°K

Minimum temperature
-55°C

Maximum temperature
+150°C

Minimum dimensions
3 x 3 x 1

Response time

Fast

Drift

Low

Self-warming
Low

Price
Low

What is a TMP121 sensor ?

The TMP121 is a digital temperature sensor with SPI from Texas Instruments, designed for low-power systems requiring stable and linear measurements.

It is a more compact and cost-effective version of the TMP125, while maintaining the same accuracy of ±1 K.

It is commonly used in:

power electronics PCBs,
embedded systems,
or portable devices requiring direct reading via SPI.

Operating principle

The TMP121 integrates an internal thermal diode whose voltage is measured and converted via a 12-bit ADC.

The result is transmitted over 16 bits via the SPI bus (read-only).

The data is encoded in two's complement (bit 15 = sign).

T(°C) : Value12b x 0,0625

Technical Specifications

Parameter	Typical value
Resolution	12 bits (0,0625 °C / LSB)
Measurement range	−55 → +150 °C
Precision	±1,0 K (typ.)
Interface	SPI (read-only, 3 wires)
Conversion time	120 ms
Max. SPI Frequency	10 MHz
Supply	2,7 → 5,5 V
Consumption	220 µA typical
Sleep mode	< 1 µA
Case	SOT-23-6 or SON-6
Annual drift	±0,1 K

Wiring Configuration

Brooch	Name	Function
1	CS	Chip Select
2	SO	SPI Output (MISO)
3	SCK	SPI Clock
4	V+	Power supply 2.7–5.5 V
5	GND	Mass
6	NC	Not connected

Self-warming

Almost none: less than 0.05 °C, even in an isolated environment.

Ideal for thermal monitoring directly on sensitive components (regulators, CPUs, RF modules).

Application areas

⚙️ Thermal surveillance of electronic components

🔋 Thermal management in embedded systems or IoT

💡 Temperature control for FPGA, ASIC, MCU

🧠 Portable or miniaturized instrumentation

🧱 Compact industrial supervision

Should I choose a TMP121 sensor ?

Strengths points

⚙️ Direct and reliable reading
→ The TMP121 provides a pre-linearized SPI value, without amplification or external calculation. Result: ultra-simple and quick integration into any microcontroller.
🔋 Low energy consumption
→ With only 220 µA in operation and < 1 µA in standby, it is ideal for battery-powered systems or those with very low thermal dissipation.
💡 Compact and precise
→ The 3 mm SOT-23-6 package allows it to be placed as close as possible to critical areas (regulators, converters, processors, etc.) while maintaining a precision of ±1 K.

Weaknesses points

🌡️ Temperature limited to 150 °C
→ The TMP121 is not suitable for high industrial temperature environments (>150 °C).
🔋 No software configuration
→ The SPI interface is read-only, with no adjustment registers (no software calibration or filters).
📶 One single measurement path
→ Single-channel sensor, so a component is needed for each measurement point, unlike an ADC type MCP3208.

Useful information

Here is some useful information regarding the TMP121 sensors.

Resolution and precision

The TMP121 provides a 12-bit signed digital output (two's complement) via SPI interface.

Each bit corresponds to a variation of 0.0625 °C, ensuring excellent measurement resolution for a miniature sensor.

Parameter	Typical value
Resolution	12 bits (0,0625 °C / LSB)
Precision	±1.0 K (from -25 to +100 °F)
Measurement range	−55 → +150 °C
Annual drift	±0,1 K
Conversion time	120 ms typical
Max. SPI Frequency	10 MHz
Power supply voltage	2,7 → 5,5 V
Typical current	220 µA
Sleep mode	< 1 µA

🔹 Remarks:

The TMP121 is factory calibrated, with no need for external correction.
Self-heating is negligible (< 0.05 °C).
The conversion is automatic upon power-up.

Application Example – SPI Reading

The TMP121 sends a 16-bit frame with each SPI read.

The 12 most significant bits represent the temperature, and the 4 least significant bits are always 0.

🔹 SPI word structure (16 bits)

Bit(s)	Name	Description
[15]	Sign	0 = positive / 1 = negative
[14 → 3]	Temperature (12 bits)	Value × 0.0625 °C
[2 → 0]	Zéro	Unused

🔹 Conversion equation

T (°C) = Valeur_{12b} × 0,0625

Practical example:

SPI reading → 0x07D0

0x07D0 = 2000₁₀

T = 2000 × 0,0625 = 125,0 °C

✅ Measured temperature = 125 °C

Integration into an electronic system

The TMP121 communicates via 3-wire SPI (CS, SCK, SO).

No adjustments or external components are required, greatly simplifying integration on the board.

🔹 Typical components

Component	Function
TMP121	SPI Temperature Sensor
Microcontroller (3.3V or 5V)	Master SPI
100 nF capacitor	Decoupling of the power supply
Pull-up resistance (optional)	Stability on CS
Supply	2,7 → 5,5 V DC

🔹 Functional diagram (ASCII)

      +3.3V / +5V
          │
       [TMP121]
          │
     SO ─────► MISO (MCU)
     SCK ◄──── CLK  (MCU)
     CS  ◄──── GPIO (MCU)
     GND ───── GND

💡 Multiple TMP121 devices can share the same SPI bus, provided each has a dedicated CS line.

🔹 Operating Principle

1️⃣ The microcontroller pulls down the CS line.

2️⃣ The TMP121 sends 16 bits representing the temperature.

3️⃣ The MCU calculates T=value×0.0625 T = value × 0.0625 T=value×0.0625.

4️⃣ The sensor returns to low power mode until the next reading.

Practical advantage

Advantage	Description
🧠 Direct reading in °C	Internal digital conversion, no calibration required
⚡ Simple and fast SPI (10 MHz)	Compatible with most MCUs
💡 Miniature case (3 mm)	Ideal for direct mounting on PCB
🔋 Low consumption	Suitable for battery-operated systems
🎯 Good stability and low noise	Accuracy ±1 K, drift ±0.1 K/year
🧱 Immediate integration	No external component required