Accurately measure vital signs with low Iq and a small form factor

These days, it feels like the “portable future” is right around the corner. Devices that used to be cumbersome and bulky have become light and portable. I saw this firsthand in personal electronics: cellphones were once heavy and slow; they’re now slim, fast and have an increasingly longer battery life.

I’ve also witnessed this trend in personal health care. It is now possible to check vital signs without having to go to the doctor, partly because of the increasingly small solution size and low power of devices such as blood glucose monitors in the palm of your hand. Blood glucose monitors are experiencing a growing trend in lower power with longer battery life that enables users to have a responsive body vital measurement device.

Blood glucose monitors are devices that have extremely low power and try to push quiescent current (Iq) to the lowest limits possible because they must be able to measure at least 1,000 tests on the same battery which is typically a lightweight 3V button cell. Reaching a battery life that can handle 1,000 tests has become an issue as blood glucose monitors start to become more connected with Bluetooth® and other wireless connectivity (as shown in Figure 1). This is because the increase in wireless connectivity in turn increases power consumption and lowers battery life and increase the need for multiple coin-cell (220mAh) or even AAA batteries (1,000mAh), which have an increase in size and weight.

Figure 1: Blood glucose monitor system diagram

Unlocking higher accuracy

As system power decreases it can become a challenge to maintain a high accuracy, so it is important to make sure that system accuracy remains high; one way to increase accuracy is to have an external voltage reference. An external voltage reference is often practical because there are several techniques – such as oversampling on an analog-to-digital converter (ADC) – that increase the requirements of the reference voltage well above what a typical internal voltage reference can do. The increased requirements on a voltage reference can be a combination of an improved initial accuracy, a lower temperature coefficient, lower noise or even a different voltage reference level. These requirements are typically difficult to achieve in a lower-power application, but the REF3320 from the REF33xx family of voltage references solves these issues by providing a high accuracy, low temperature coefficient at a low Iq.

A low Iq voltage reference

The REF3320 is one of TI’s low-power precision voltage references. The largest advantage of the REF33xx family is its typical 3.9µA low supply current requirement and its ability to source and sink up to 5mA for an ADC or digital-to-analog converter (DAC) while active. This allows the REF3320 to have a very minor impact on overall system power while the system is sampling, as shown in Figure 2.

Figure 2: Total power consumption overhead example (estimated percentage)

The REF33xx family also offers low-voltage options between 1.25V (the REF3312) and 3.3V (the REF3333) to benefit applications that use coin-cell batteries. These output voltages offer you the flexibility to get the most out of your ADC by selecting an adequate voltage reference to take advantage of the complete full-scale signal. Higher output voltages also give you the flexibility to power ultra-low-power ADCs such as the ADS7042 that rely on using AVDD as the voltage reference.

Low Iq is possible in more ways than one

TI’s large, low-power voltage reference portfolio includes several options aside from the REF33xx family. For certain applications that need the absolute lowest power for regulation, the 1.25V REF1112 has a typical power consumption of 1µA in a small package, making it TI’s lowest-power voltage reference.

But there are more ways to save power. One such way would be to use the enable feature of the REF3425to limit power consumption while the device is active. This is a feature in TI’s REF3425, which is a high-precision voltage reference that can achieve 2.5µA of Iq in shutdown mode. It is also possible to use a load switch to turn of sections of the system to lower Iq which can further bring down the standby current of a blood glucose monitor. Figure 3 shows the power consumption of several popular low-power voltage references. 

Figure 3: Typical Iq of several TI voltage references

1.5mm-by-1.5mm UQFN voltage reference

The REF3320 or REF1112 also shine in continuous-sampling low-power monitors such as gas analyzers, personal radiation detectors and smoke detectors. These low-power applications continuously sample every minute (up to several hundred samples) while still maintaining a small battery and form factor. For example, due to the harsh temperature conditions in radiation detectors, the REF3320 has a low temperature drift of 30ppm/°C from -40°C to 125°C that ensures an accurate reference across temperature. In addition, the REF3320 is available in a small-form-factor 1.5mm-by-1.5mm UQFN package, as shown in Figure 4. This small form factor also gives you the flexibility to add passives for additional noise filtering and still be smaller than a typical SOT23-3 package.

Figure 4: The REF33xx family in a UQFN package

Don’t force yourself to choose between low system power and high system accuracy, as it is possible to increase ADC and DAC accuracy by adding a low-power voltage reference like the REF3320 or REF1112. No matter which application you are designing for, TI offers a large portfolio of voltage references that can help you unlock more accuracy in your system.

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