A Configurable Sensor Monitor for Power-Sensitive Designs from Texas Instruments

Texas Instruments recently announced a new nanopower, dual-channel, programmable sensor monitor to help designers optimize system power.

The ADS7142 from Texas Instruments is a programmable sensor monitor that is physically small (1.5 × 2 mm), has a configurable dual-channel analog multiplexer input, and is touted as being an ultra-low-power device.

 

Figure 1. TI's ADS7142 sensor monitor is available in a 10-pin, 1.5mm × 2mm package. Images courtesy of the datasheet (PDF) and TI.com.

 

Although this sensor can, according to its datasheet, operate as a standalone device—and it could indeed function in the absence of a microcontroller in lower-complexity applications—it appears that this sensor monitor could really shine when used in conjunction with a microcontroller, specifically one that functions as a wireless microcontroller, such as TI's CC1310 SimpleLink Sub-1 GHz MCU, or one that is otherwise a power hog.

The ADS7142 is described as a nanopower device because it's designed to autonomously monitor sensor outputs while using only 900nW of power. If the ADS7142 were designed to monitor sensors in a power-sensitive design, such as an IoT application or a battery-operated system (think wearables), the primary microcontroller could spend more time in its low-power mode.

The designer would have to "run the numbers" on power consumption to determine if the ADS7142 could save enough power to justify the added cost. For example, if a microcontroller spends most of its time in a rather high-current active mode, performing various calculations in addition to gathering sensor data, then pairing the microcontroller with a separate sensor monitor would probably not make sense in terms of power savings. But if the microcontroller comes out of sleep mode once a day or once an hour (and only for a few seconds) to merely gather sensor data, then using the ADS7142 in conjunction with the microcontroller may prove to be a beneficial approach.

 

Figure 2. An example of a system architecture in which the ADS7142 could be used, taken from the datasheet (PDF).

 

Some possible applications (mentioned in the datasheet) in which this sensor monitor could be used in conjunction with a microcontroller are the following: gas, heat, and passive infrared motion and smoke detectors; sensors for deep learning artificial intelligence; and preventative maintenance for industrial equipment.

Optimizing Power Consumption

Section 7.5 (Optimizing Power Consumed by the Device) of the datasheet offers some guidance on how to configure the device for optimal power consumption. These tips include the following:

• Ensure that the analog power supply (AVDD) is as close as possible to the analog input voltage.
• Maintain the lowest allowable voltage on the digital voltage supply (DVDD).
• When in manual mode (see definition below), operate the device at the optimal sampling rate.
• When operating in autonomous and high-precision modes, achieve optimal balance between sampling speed and power consumption by "selecting the oscillator for conversion and setting the nCLK value." (For some reason, this last suggestion comes across as both obvious and nebulous at the same time.)

As defined/described in the datasheet:

• Manual Mode allows the external host processor to directly control when the data is sampled. It sounds like this is not the ideal mode for achieving maximum power savings.
• Autonomous Mode occurs when the device is programmed to monitor its analog input pins and, by means of the ALERT pin, inform the host when the programmable high or low threshold values are crossed. This mode appears to be the one that makes it possible to achieve the ultra-low 900nW power consumption.

The plots below demonstrate steps that can be taken to minimize the ADS7142’s current consumption.

 

Figure 3. SCL and nCLK can be adjusted to reduce the current consumption of the ADS7142. Plots taken from the datasheet (PDF).

Configurable Analog Input and Multiplexer

The input multiplexer of the ADS7142 can be set to one of the following configurations:

• two-channel, single-ended
• single-channel, single-ended with remote ground sensing
• single-channel, pseudo-differential channel

The two-channel single-ended configuration is the sensor's default configuration after power up. In this mode, each channel can accept analog input signals ranging from 0V to AVDD (1.65 to 3.6 V). See the figure below.

 

Figure 4. The ADS7142 configured for two-channel, single-ended input. From the datasheet (PDF).

 

When configured as a single-channel single-ended input with a remote ground, the device can monitor analog signals ranging from 0V to AVDD, and the remote ground sense can accept signals ranging from -100mV to 100mV (see image below). As described in the datasheet, this type of configuration is handy when the sensor itself is located far from the sensor monitor, such that minor differences between the two ground potentials could be present. See the figure below.

 

Figure 5. The ADS7142 can be configured for a single input channel with remote ground. Taken from the datasheet (PDF).

 

The single-channel, pseudo-differential configuration can be useful when monitoring sensors such as electrochemical gas sensors, which produce signals like the ones shown in the following diagram.

 

Figure 6. The ADS7142 can be configured for single-channel, pseudo-differential input. Diagram taken from the datasheet (PDF).

 

Do you have any experience with this product or similar sensor monitor ICs? If so, feel free to share your thoughts in a comment.