Introducing an Ultra-Low Power, DC Coupled Input Signal Chain
Now that we gave an AC coupled example (AD8233) in our previous blog post, the recent release of the AD4130-8 is a great opportunity to discuss a DC coupled input signal chain. If you recall in the second blog, we mentioned that DC coupled signal chains dealing with large sensor offsets require higher resolution ADCs that typically burn more power and take additional board area.
With the AD4130-8 shown in Figure 1 you have an unparalleled depth of signal chain integration in ADI’s unique architecture, which delivers a 24-bit Sigma-Delta ADC complete with integrated Programmable Gain Amplifier (PGA) and FIFO at an impressive 32uA quiescent current in continuous conversion mode. Having a supply voltage range of 1.71-3.6V and a tiny 2.7mm x 3.56mm WLCSP footprint, the AD4130-8 is ideal for battery powered applications such as field instruments, smart transmitters, and wireless sensor nodes, where temperature and bridge (pressure, load, strain) based sensors are supported. Additionally, the integrated crosspoint multiplexer brings added flexibility by enabling a measurement channel between any pair of the 16 analog inputs.
Figure 1 – Block Diagram of AD4130-8
Low Power at the System Level
When describing precision low power signal chains in the first blog, we mentioned a system level approach is needed to truly optimize battery life. The AD4130-8 does just that by incorporating features that consider the power of the complete solution from the sensor to the microcontroller. Let’s first think about a typical 1kΩ bridge sensor biased at a 3V reference voltage. This alone burns 3mA which is already ~100X more than the AD4130-8 that is measuring it! To remove this power “hotspot”, the AD4130-8 includes a low-side power switch (see Figure 2) that may be used to power down the bridge sensor between conversions. This low-side switch is controlled by the smart channel sequencer to tailor timing and optimize energy savings. Another option to consider for power sensitive applications would be to bias the bridge sensor with an excitation current instead of a voltage reference. The AD4130-8 includes register programmable, precision excitation current sources that range from 100nA to 200uA and can be driven off chip through any of the multiplexer input channels.
Figure 2 – Simplified AD4130-8 Bridge configuration using low-side power switch (PSW)
The on-chip FIFO (First In, First Out) buffer can store up to 256 conversion results from the AD4130-8, which further reduces the system level power burden by allowing the microcontroller to be in sleep mode for longer. A convenient interrupt signal is then used to wake up the microcontroller when the data exceeds a specified threshold indicating the FIFO has reached a predefined number of samples (Watermark Mode), or the FIFO is full and allows a “burst mode” data transfer. This, combined with the smart sequencer on the AD4130-8 enables autonomous measurements to be made.
A DC Coupled Solution for Biopotential Measurements
While it may not be obvious, the AD4130-8 is a great example of a DC coupled solution for making single lead biopotential measurements at low enough bandwidth levels such as heart rate or even ambulatory ECG. Figure 3 shows a basic configuration where two analog inputs are tied to LA (left arm) and RA (right arm) electrodes, and a third analog input is used to drive the body to a DC Vbias of AVDD/2. While this is not a true Right Leg Drive (RLD) it may be sufficient for battery powered solutions. Additionally, burnout currents can be used as a spot check for DC lead off detection. Note that with the available burnout current levels, these are better suited to lower impedance/wetted electrodes. For dry electrode applications where the input bias currents of the AD4130-8 are too high, buffers such as the ADA4505-2 or MAX40024 could be placed at the electrodes prior to the AD4130-8.
Figure 3 – Single Lead ECG configuration for the AD4130-8
Since this is a DC coupled input signal chain, the supply voltage and expected electrode offset must be considered when setting the gain of the PGA to prevent saturation. A table like the one shown in Figure 4 (Table 15 of the AD4130-8 Datasheet ) is a helpful way to track the noise for a given gain setting and Output Data Rate (ODR). The ODR also sets the 3dB bandwidth for the application as shown in the third column. The AD4130-8 configuration has also been tested against the IEC60601-2-47 specification for ambulatory ECG and was able to pass all requirements assuming an accurate enough external clock is used for the timing accuracy test.
Figure 4 – Noise vs Gain and Output Data Rate for the AD4130-8 with 2.5V Reference and Sinc3 Filter
In the next blog we’ll go into more detail on Common Mode Rejection Ratio (CMRR) and the Right Leg Drive concept when making Biopotential Measurements.