Low-Noise Amplification for Low Current Signals

There are a plethora of mobile fitness and health monitors on the market today, and the demand for them does not seem to be slowing down. Most of these devices are small mobile devices that are designed to use very low power to maximize battery usage. Additionally, some of the sensor outputs for these health monitors are very low, which makes it imperative to use electronic components that add as little noise to the circuit as possible.

Take for example a photodiode used in heart rate monitoring (HRM) or pulse oximeter oxygen saturation (SpO2) applications. These photodiodes typically only provide a signal of 10 µA or less which needs to be amplified to make use of it. Therefore, the amplification process needs to add as little noise as possible in order to preserve the integrity of the original signal and to maintain accuracy.

With this in mind, Analog Devices has designed a low noise current amplifier for smaller photodiodes: the ADPD2210. This device amplifies sensor signal currents 24 times the input current while adding very minimal noise, providing the large photodiode system sensitivity along with the benefits of using a smaller photodiode. The 60 dB minimum linearity of the ADPD2210 allows for accurate extraction of very small time variant signals on top of large DC or low frequency offsets.

Analog Devices' EVALZ-ADPD2210 Evaluation Board

The ADPD2210 is optimized for pulse mode applications such as wrist worn HRM or finger worn pulse SpO2. These applications require low power consumption and rejection of ambient light which could cause additional errors to the measurement process. The ADPD2210, in photodiode applications, holds the sensor input to within ±5 mV (typical) of the reference terminal. This provides near zero-bias voltage and allows minimal dark current and shot noise to affect the system accuracy.

Power conservation is critical in battery powered applications such as these. With that in mind, the ADPD2210 was designed to consume as little power as possible while offering exceptional performance. It typically uses 140 μA with no input and up to 954 μA at full scale. Additionally, the ADPD2210 can be put into a standby mode via a power down pin (PWDN) when sensing is inactive. Doing this will add critical time for battery-powered monitoring and can reduce disposable application battery costs.

Sensor site amplification using the ADPD2210 reduces the effect of electromagnetic interference (EMI) in low level wired interfaces, provides improved signal-to-noise ratio (SNR), and offers rejection of interference signals from nearby equipment. This combination of low power, high SNR, and EMI immunity enables low power system solutions not possible with traditional small current sensors.

The output of the ADPD2210 can be used in either a 3-wire voltage or 3-wire current configuration. The 3-wire voltage configuration allows flexibility in matching the analog-to-digital converter (ADC) full-scale input to the full-scale output of the ADPD2210. When used in a 3-wire current configuration, the ADPD2210 is insensitive to load resistance and can be used when the signal processing is further from the sensor.

In addition to HRM and SpO2 photodiode applications, the ADPD2210 can be used in any application that requires ultralow noise amplification of small current. An evaluation board (EVALZ) is available to determine if the ADPD2210 would be suitable in a design under consideration.