White PaperFlexible Temperature Measurement with the ZSSC3241 Enables
Bridge as Temperature Sensor

Introduction
Accurate measurement using sensor elements requires precise and local measurement of the element’s
temperature for correction and compensation across a wide operating range. Thus, an integral part ofSensor Signal Conditioner (SSC) functionality is to provide at least one or more methods to measuretemperature of the sensor element, which is typically a resistive bridge or capacitive transducer.
One simple and effective way to create a precise thermometer is to build a standard circuit called a PTAT
(Proportional To Absolute Temperature) that generates a linear, consistent voltage versus temperature.
This is fine as long as the SSC silicon is tightly coupled thermally to the sensor element such that thetemperature of the two elements is nearly equal.
In some cases, however, the sensor element may be separated from the SSC by several centimeters oreven further. In these instances, the SSC cannot be used predictably for temperature compensation andsome type of external thermometer is needed that can be co-located with the bridge sensor. Silicon diodes
or thermistors of the NTC or PTC type are commonly used for these cases, but these solutions require
additional components and may have the same issue of co-location with the primary sensing element.
For resistive bridges, the ultimate way to measure the sensor temperature accurately with no additional
cost or space is to use the temperature coefficient of the bridge itself. However, this requires some
additional complexity in the biasing circuitry for the sensor bridge and more sophisticated calibration
algorithms in the SSC to linearize the temperature coefficient of the bridge, which may exhibit up to thirdorder variation.
With advanced modern SSCs like the ZSSC3241, this functionality is fully integrated and supported. TheZSSC3241 can support temperature measurement using an internal PTAT, external diodes or thermistors,
and using the main sensing bridge itself as the thermometer. The various options are fully programmableand configurable using internal Non-Volatile Memory (NVM) for optimal performance based on system
requirements.

The ZSSC3241 provides for 7 different temperature measurement
modes
Below is the basic block diagram for the ZSSC3241, showing the configurable sensor interface with biasing
and amplification functions in the analog frontend. There is also an Internal Temperature Sensor onboard.
The ADC converts the sensor and temperature readings into the digital domain, where all the linearization,correction, and temperature compensation functions are performed. The final calibrated and scaled outputs
are available as either analog or various digital representations.

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Flexible Temperature Measurement with the ZSSC3241 Enables Bridge as Temperature Sensor

Figure 1. Block Diagram of the ZSSC3241 Sensor Signal Conditioner (SSC)

Looking more closely into the analog frontend, there are four blocks in particular that enable the ZSSC3241
to reconfigure temperature measurements: a sensor-supply multiplexer to switch between current andvoltage bias modes, a sensor bias current source, internal temperature sensor, and an input multiplexer
that selects between the measurands. These blocks are controlled by bits in the NVM labelled sensor_sup,Tbiasout, and gain_polarity.

Figure 2. Configurable blocks of the Analog Front End interface to the bridge and temperature sensors

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Flexible Temperature Measurement with the ZSSC3241 Enables Bridge as Temperature Sensor
With these blocks and the configuration options, the ZSSC3241 provides for 7 different temperature
measurement modes as summarized in the table below. There are two primary biasing modes for using thesensor bridge as a thermometer: current or voltage. For each of these types a series resistor, Rt, isrequired to adjust the common mode voltage of the bridge into the proper range for the internal
Programmable Gain Amplifier (PGA). The ZSSC3241 provides the option to have this series resistancefully integrated for many cases, but some external resistance might be required in cases where accuracy
requirements are stringent.

Reference

Temperature
SensorBridge bias
mode

External
components

Current/Voltage None

Comments

T0

Internal PTAT

Independent bridge bias

T1

Sensor Bridge

Current

None

Internal Rt for CM adjustment

T2

Sensor Bridge

Current

Resistor(s)Ext. Rt should have low tempco

T3

Diode/Thermistor

Voltage

Diode/Thermistor

No series resistors

T4

Sensor Bridge

Voltage

None

Internal Rt for CM adjustment

T5Sensor Bridge

Voltage

Resistor(s)Ext. Rt should have low tempco

T6

Diode/Thermistor

Voltage

Diode/Thermistor

Same as T3 with internal Rt

Table 1. Summary of Temperature sensing options in the ZSSC3241

The figures below provide more details on how the blocks are configured for each of the cases in the table.
In each figure, the blue lines show the bias path for the sensor bridge and the red lines show the signal
path measured to represent temperature.

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Configuration for using internal PTAT thermometer

Figure 3.

Figure 3 shows the configuration for T0, using an internal PTAT thermometer. Sensor bridge biasing iscompletely independent and can be either ratiometric voltage or current. No external components arerequired.

Configuration for using the sensor bridge as a thermometer

Figure 4.

Figure 4 illustrates the configuration for T1, using the sensor bridge as a thermometer. In this case the bridge
is driven by current and two internal resistors, Rt and Rt’, are used to adjust the Common Mode voltage ofthe bridge into the appropriate range for the ZSSC3241. The values of Rt and Rt’ are adjustable by settings
in the NVM registers.

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Flexible Temperature Measurement with the ZSSC3241 Enables Bridge as Temperature Sensor

Configuration with external Rt for cases with high accuracy requirements

Figure 5.

Figure 5 shows configuration for T2 with external Rt for cases with high accuracy requirements. Rt,external
should have a low temperature coefficient so that the temperature variation is dependent on the sensor bridgetemperature coefficient. The internal Rt’ is optional depending on whether CM adjustment is required.

Configuration with external diode or thermistor as thermometer

Figure 6.

Figure 6 illustrates the configuration for T3 with external diode or thermistor as thermometer. The thermistor
can be of the NTC or PTC type. The temperature sensor is biased with a programmable current source andthe bridge sensor is voltage driven. Internal Rt’ can be used if necessary for shifting the CM voltage.

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Flexible Temperature Measurement with the ZSSC3241 Enables Bridge as Temperature Sensor

Configuration using the sensor bridge as a thermometer with internal Rt andRt’

Figure 7.

Figure 7 shows the configuration for T4, using the sensor bridge as a thermometer with internal Rt and Rt’.
This corresponds to T1 except the bridge is biased by voltage rather than current.

Figure 8.

Figure 8 shows the configuration for T5, using the sensor bridge as a thermometer with external resistance,Rt, external. This corresponds to T2 except the bridge is biased by voltage rather than current.

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Configuration using an external diode or thermistor for temperature sensing

Figure 9.

Figure 9 illustrates the configuration for T6, using an external diode or thermistor for temperature sensing.
This corresponds to T3 but allows for a high-side resistance, Rt, internally in case additional CM voltage
adjustment is required for the external temperature sensor.

Summary
Additional information for these configurations including the range of currents, voltages, and Rt/Rt’ values
along with the corresponding NVM settings is available in the datasheet for the ZSSC3241. The references
T1-T6 used here correspond to the labels used in the datasheet.
With its wide range of front-end configurations, the ZSSC3241 enables designers to optimize both the main
sensor interface and the type and location of sensors used for temperature compensation in order toachieve the best accuracy possible with fewer components and cost.
Renesas also provides additional resources for developing and characterizing designs with the ZSSC3241.
These include a full evaluation kit with device samples, application notes, and dedicated support staff.

Reference■ZSSC3241 product page: renesas.com/zssc3241

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Flexible Temperature Measurement with the ZSSC3241 Enables Bridge as Temperature Sensor

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Doc Number: R36WP0001EU0100

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