The incredible versatile op amp in medical apps

The incredible versatile op amp in medical apps

The op amp enables designers to solve a multitude of problems, and its uses are limited only by the imagination

BY JOHN ARDIZZONI
Analog Devices, Norwood, MA
http://analog.com

Operational amplifiers (op amps) are a staple in modern electronics. Found in everything from industrial flow metering to ultrasound imaging, versatility is their appeal. This building block is unique in that no other single integrated circuit (IC) can be used in so many different applications and configurations. Besides providing amplification, op amps can realize a wide range of functions such as summing, buffering, subtraction, integration, differentiation, filtering, inversion, and current-to-voltage conversion, to name a few. This article focuses on medical applications, but could easily cover industrial, consumer, or communications, as the same fundamental op amp building blocks are common to a variety of applications. We’ll also discuss “specialty op amps” and new functionality that can be found in today’s op amps.

Healthcare products are no longer restricted to hospitals and clinical settings. Today’s consumer can purchase a wide variety of home healthcare electronics including automatic blood pressure monitors, fingertip pulse oximeters, digital thermometers, and blood glucose monitors. Op amps, used in medical platforms that include diagnostics, therapy, monitoring, imaging, and instrumentation, can be found in almost any block diagram. In the main signal path, secondary circuits and power supplies, their uses are countless.

Let’s take a close look at a low-power portable pulse oximeter. Then, at a higher level, we’ll discuss a few other applications and the common op-amp building blocks they share.

Pulse oximetry

A pulse oximeter is a noninvasive device that measures the percentage of hemoglobin saturated with oxygen (SPO2) and the pulse rate of a patient. Typically clipped to a fingertip or earlobe, the pulse oximeter can be used in a variety of settings: operating rooms to monitor oxygenation and pulse rate while under anesthesia, during machine assisted ventilation, and as an added safety measure during outpatient procedures. Numerous op amp circuits are used in the pulse oximeter. These same building blocks are widely used in other medical and non-medical applications. Figure 1 shows a typical finger tip pulse oximeters.

Fig. 1. Fingertip pulse oximeter.

The pulse oximeter works by measuring the light absorption or reflection of Hemoglobin, which absorbs light differently when carrying oxygen (oxyhemoglobin) than when not (deoxyhemglobin). Red (R) and infrared light (IR) sources are used along with a photodiode detector to measure the amount of light that passes or reflects through the tissue sample. Hemoglobin saturated with oxygen will absorb IR, while hemoglobin carrying low levels of oxygen will absorb red light. The R/IR ratio is compared to a lookup table to yield the SPO2. A typical SPO2 of 0.5 indicates approximately 100% SPO2, while a ratio of 2 indicates 0% SPO2.

Fig. 2. A typical block diagram for a pulse oximeter consists of numerous op amps and other components used in this application.

Figure 2 shows a typical block diagram for a pulse oximeter. While this may appear to be relatively simple application, the block diagram shows that there are numerous components. Op amps are used to convert current to voltage (I to V), drive an ADC (analog-to-digital converter), drive LED excitation sources, buffer a microprocessor input, and process audio. Many of these op amp building blocks can be found in just about any electronic device.

The pulse oximetry light is provided by two light emitting diodes (LED). A red LED emits light in the 600-nm (nanometer) to 700-nm region, while an IR LED emits in the 800-nm to 900-nm region. The two LEDs are switched on and off at a high rate by the current sources formed by U1 and U2. The transmitted or reflected light is detected by photodiode D3, which produces a small current proportional to the light detected. This current is converted to a voltage via a precision FET-input op amp that is typically used in photodiode detectors or current-to-voltage converters. FET-input op amps require very little input bias current, ensuring that the majority of the photodiode current passes through the feedback resistor, providing accurate data.

The output of the I-to-V converter is then filtered, either actively or passively; an active filter provides another opportunity for an op amp to be used. The signal is then fed to a buffer op amp, which interfaces to the microprocessor to indicate a no-connection fault if the pulse oximeter falls off the patient.

The signal is also fed to ADC driver U5, which sets the signal to the appropriate amplitude and offset settings, which are compatible with the ADC. Depending on the system requirements, the ADC driver can be realized with traditional op amps or a differential amplifier, which is a special class of op amps that we’ll discuss in more detail later. In this portable application, a low-power op amp is most appropriate. The signal is then digitized and analyzed by the microprocessor.

U1 and U2, precision micropower op amps, form the heart of the current sources that drive the red and IR LEDs. The two LEDs require different currents, so a dual op amp is a smart choice to save board area and lower cost. The voltage from the DAC (digital-to-analog converter) is fed to U1 and U2 via analog switch SW1. The op amps have limited output current capability, therefore external FET pass transistors (Q1 and Q2) are used. The voltage across RISET1 and RISET2 sets the current through the LEDs.

The op amp U6 provides the drive signal to a speaker or transducer for audible monitoring purposes. This application is a good example is of where and how op amps can be used in medical electronics. Though physically small, the pulse oximeter puts a lot of electronics right at your fingertips!

No matter whether the application is digital X-ray, industrial flow metering, white goods, or flow cytometry, the same op amp building blocks can be found in all these applications, as shown in Fig. 3 . Flow cytometry, illuminates single cells with a laser and then detects the scattered light. The scattered light signature tells the story of the cell condition. In digital X-ray applications that use indirect conversion, the X-ray radiation must first be converted to light via a scintillator, which is then fed to a photodiode detector. Both the flow cytometry and digital X-ray use a photodiode detector (I to V converter) and then send the signal to an ADC, first passing through an ADC driver. This pattern is repeated for just about any signal acquisition chain, the main difference being how the signals are captured.

Fig. 3. Flow cytometry block diagram.

Specialty op amps

Specialty op amps, derivatives of traditional op amps, are used in many applications including medical. Let’s take a look at a few.

Differential amplifiers are a special class of op amps; they are ideally suited for driving high-speed and precision ADCs and for driving video signals over unshielded twisted pair (UTP) cables. Similar to traditional op amps, differential amplifiers can process differential or single-ended signals at their inputs, but differential amplifiers have balanced differential outputs while op amps have a single-ended output. Differential amplifiers also have a dedicated pin that controls the output common-mode voltage. If a system uses an ADC, it’s likely that a differential amplifier drives it; with a DAC, a differential amplifier often buffers the output.

Instrumentation amplifiers (in-amps) are used as a key interface between patient and equipment, as they have very high input impedance, and low input offset and bias currents. The differential inputs are balanced so that the input source and output can be independent of any load reference. In-amps also have very high common-mode rejection, so that coupled noise pick up and ground drops are kept to a minimum. Instrumentation amps can be found in ECG and EEG machines.

Variable-gain amplifiers (VGA) and programmable-gain amplifiers (PGA) can be lumped into one category. These amplifiers, which do not require feedback or gain-setting resistors, can change their gain with an analog voltage or a digital interface. Their input and outputs can be either single-ended or differential. VGAs or PGAs are commonly found in automatic gain-control (AGC) applications, wide-dynamic-range data-acquisition systems, equipment that uses photodiode circuits, and ultrasound front-ends.

Package, power, and pinout

Portable electronics must be small size and low power, as their area and current budgets are extremely small. The wafer-level chip-scale package (WLCSP), the smallest available, actually eliminates the package. As shown in Fig. 4 , a dual low-power op amp, the package is the die itself with bumped eutectic solder dots. This minimizes board area and reduces the effects of parasitics. One drawback to this package option is the limited number of parts offered in it. Traditional small packages such as SOT23, SC70, LFCSP, and DFN are still great choices. Some of the new packages also feature an exposed paddle, which lower the package thermal resistance and increases reliability.

Fig. 4. Wafer-level chipscale.

Typically powered by batteries, the current budget for portables is small. Therefore, low supply voltages and currents are critical. Some new op amps dissipate only a few hundred nanowatts of power. When discussions turn to body area networks (BAN) or WBAN (wireless BAN), power is of the utmost concern. In BAN applications, the user wears devices that collect body data at a local level. Some work has already been done to power these devices from body heat, with a thermoelectric generator (TEG) worn on the users wrist to convert body heat to voltage. Now that is low power!

The traditional op amp pin-out has been around for over 40 years, with virtually no change until recently. Analog Devices first introduced a new high-performance pin-out with a dedicated feedback pin, as shown in Figure 5 . The new pin-out can be found on some of the company’s newest high-speed amplifiers and differential amplifiers.

Fig. 5. Dedicated feedback pinout.

The new pin-out is essentially the same as the traditional op amp pinout, except that pins have been rotated counterclockwise (CCW) one position. The CCW shift provides two benefits. First, it allows for all the input and outputs to be on one side. This greatly simplifies the layout and reduces board parasitics. Second, mutual coupling that occurs between pin 3 (non-inverting input) and pin 4 (–Vs) in the traditional pinout leads to degraded second-harmonic distortion. By rotating the pinout one pin CCW, the coupling is broken. Improvements to second-harmonic distortion can be significant; a gain of greater than 10 dB between traditional pinouts and the dedicated feedback pin is not uncommon.

New trends in op amps

Today’s op amps are being asked to deliver better performance, often with lower supply voltage and current, and new amplifiers consume only nW of power. Op amps with higher levels of integration are now starting to emerge. Some now feature charge pumps and inverters. The charge pumps can be used in the front-end of the amplifier to enable wide swings, lower distortion, and improved common-mode rejection (CMR). Other charge pumps are used with inverters to generate negative supplies. This provides a huge savings in single-supply applications, allowing the amplifier output to swing below ground on a single supply and saving the cost and board space required for a negative voltage. Other features such as serial interfaces, selectable filters, muxes, and load detectors can now be found on op amps.

Op amps are a key building block and pervasive in so many different applications and systems. Regardless of the application, there’s bound to be an op amp in it. In the future op amps will be asked to deliver higher performance; changes are already showing up, very lower power, higher levels of integration and additional functionality and features can be seen today. The versatility of the op amp enables designers to solve a multitude of problems, and its uses are only bounded by the imagination.