Written by: Greg Davis, Sr. Product Marketing Manager for Microchip Technology’s Mixed Signal Linear Business Unit
The need for instrumentation amplifiers
Instrumentation amplifiers (INA) are based on the same architecture as op amps. We could say that it is a somewhat specialized version of an op amp. Due to their high differential gain, they are used in amplification of micro-volt level sensor signals. At the same time, we can also reject high voltage common-mode signals. This is very useful in cases where we need to accurately capture a relatively small change in voltage or current, such as that produced by some sensors.
INAs have several applications. For example, it’s present in a medical instrument used in vision correction eye surgery, where high accuracy is crucial. The instrument uses sensors to align laser stepper motors. If sensor signals are compromised in the equipment in the operating room, it can cause unexpected results. INAs are also used in factory presses. These machines bend metal into shapes by using thousands of pounds of force. To ensure safety, the machines use sensors that are designed to bring the machine to a stop in case it detects a human hand. The system could malfunction if there is interference from electrical noise from other equipment, leading to dire consequences. Therefore, the signal needs to be detected accurately from the sensors.
Both the above instances demand that tiny sensor signals must be accurately amplified in all environments, which is exactly what instrumentation amplifiers are designed to do.
Some things to consider while using INAs. Low power consumption allows the battery to be used over more of its depletion curve, thereby extending its life. Also, for compatibility with more sensors, a wide input voltage range is recommended, along with impedance matching at the input.
How INA designs evolved
Given the performance benefits that INAs offer, they have evolved to find applications across consumer, medical and industrial segments. There are several performance improvements seen in present-day instrumentation amplifiers, compared to original approaches. Here’s a look at some of the approaches.
To set the context, the diagram in Figure 1 gives us a context into what we are trying to accomplish.
The INA inputs that amplify the differential voltage are connected to the sensor outputs. There are several sources of noise, both radiated and conducted. Switching power supplies, motors and wireless devices are some of the typical noise sources. Shielding and good PCB layout practices can help mitigate some of the noise, although some noise still gets through. Since most of that noise shows up as an in-phase, we can reduce this voltage and maintain good accuracy by superimposing common-mode voltage (VCM) on the differential input sensor voltage (VDM), and by using a properly designed instrumentation with good Common-Mode Rejection Ratio (CMRR). A minimum CMRR is typically specified at DC, while the AC CMRR performance is documented in performance curves.
The discrete difference amplifier
A simple difference amplifier works if you need to amplify the voltage difference across the sensor output. There are a lot of downsides, however. For example, if we refer to in Figure 2, which shows a simple implementation, VIN+ is biased to VREF (typically ½ the supply voltage) for single-supply applications. The operational amplifier is designed to amplify differential voltages and also provides good CMRR, but the circuit surrounding it can often overwhelm it. The ability of the op amp to reject common-mode signals is limited by any mismatch in the external resistors (including mismatch contributed by any divider network connected to VREF) limits. This results in reduced CMRR. It not possible for resistor tolerances to maintain a good CMRR that one expects from an INA are simply not tight enough. The equations below show how much the resistor mismatch affects CMRR.
This equation uses a difference amplifier with G = 1V/V, and TR is the resistor tolerance,
- If TR = 1%, worst case DC CMRRDIFF will be 34 dB
- If TR = 0.1%, worst case DC CMRRDIFF will be 54 dB
Where ‘K’ is the net matching tolerance of R1/R2 to R3/R4
K can be as high as 4TR (worst case)
The amplifier amplifies the differential voltage at the input, and the gain of the amplifier is:
VOUT = G * VDM
= (R1/R2) * (VIN+ – VIN-) + VREF
The issue arises because the differential voltage (VIN– and VIN+) includes superimposed noise. Therefore, the common-mode voltage that is not rejected (as a result of poor CMRR) will be amplified. This will result in the output being corrupted by noise.
There are other drawbacks too. Even the input impedance of an operational amplifier is typically high (MΩ to GΩ range), it gets both reduced and imbalanced due to the feedback path and reference. This loads the sensor and adds to the inaccuracies. Because of the poor gain accuracy in the presence of noise, this circuit is not useful for instrumentation purposes.
Three operational amplifier IC
The three operational amplifier IC is essentially a common INA packaged in a single integrated circuit (IC). The circuit is divided into two stages. The input stage consists of two inverting buffer amplifiers, while the output stage is a traditional difference amplifier. Trimmed resistor semiconductor designs make it possible to the internal resistors used throughout this design to a very close tolerance; resulting in a much higher CMRR. Loading of the sensors is minimized by the high impedance that the input stage amplifiers provide. The designer can select any gain within the operating region of the device (typically 1 V/V to 1000 V/V), using the gain-setting resistor, RG. The output stage is a traditional difference amplifier. The gain of the internal difference amplifier, which is typically G = 1 V/V for most instrumentation amplifiers, is set by the ratio of internal resistors, R2/R1. The balanced signal paths from the input to the output yield excellent CMRR. System costs are lower since the design has a small footprint and fewer components, and is simple to implement. The design is also compatible with single-source supplies using the VREF pin. There are some limitations, however, even with this design. The feedback architecture can substantially degrade the AC CMRR given that the three op amp INAs achieves high CMRR at DC by matching the on-chip resistors of the difference amplifier. In addition, this is an issue of mismatches and reduced CMRR over frequency due to parasitic capacitances that cannot be completely matched. The common-mode voltage input range is limited to ensure that internal nodes do not saturate. The VREF pin needs a buffer amplifier for optimal performance. Also, since the temperature coefficient of the external and internal gain resistors is not matched, it results in a decline in CMRR.
Mathematically the gain accuracy depends on resistor matching:
Indirect current feedback
The Indirect Current Feedback (ICF) INA is made up of two matched transconductance amplifiers, GM1 and GM2, and a high-gain transimpedance amplifier (A3). It brings a novel approach to the voltage to current conversion process and does not rely on balanced resistors. Manufacturing cost is lower since it does not require internally trimmed resistors. Also, it does not require that the external resistors match any of the on-chip resistors. To ensure minimal gain drift, however, it is important that the RF and RG external resistors temperature coefficients match as closely as possible.
Increased frequency does not cause the AC CMRR to decrease significantly. Also since amplifier GM1 rejects common mode signals, IDC CMRR is high. The input range for the three operational amplifier is limited to avoid internal node saturation. By contrast, the ICF has an expanded range of operation since output voltage swing is not coupled to the input common-mode voltage. The second stage (GM2 and A3) differentially amplifies the signal and further rejects common-mode noise on VFG and VREF. For single-supply operation, we need to apply a bias to VREF.
The ICF INA gain is:
Where VDM is the differential mode voltage
Figure 5 shows a few typical applications for an INA. It shows a variety of sensors that are accurately amplified by an INA that feeds a converter and microcontroller.
There has always been a need to amplify small signals in the presence of noise; although that need has evolved with time. Approaches such as the discrete operational amplifier and integrated three operational amplifier have some disadvantages when used for this function. The discrete operational amplifier, despite being the simplest approach, is not suitable as an INA. The integrated three operational amplifier approach does have some significant advantages such as high DC CMRR, balanced and high input impedances with one gain resistor. At the same time, it has limitations around common-mode voltage and difficulty in matching internal versus external resistor temperature coefficients, resulting in gain drift. Additionally, there can be negative impact of impedance at the VREF pin on CMRR unless a buffer is used. The ICF approach too has a high CMRR (even at higher frequencies), a wider common mode voltage range and no on-chip trimmed resistors resulting in lower cost and low temperature coefficient gain drift. With INAs, designers have an excellent method to amplify micro-volt level sensor signals. At the same time, the INAs reject high common-mode signals found in noisy environments.