A closer look at special function amplifiers

Have you ever wondered why there are so many variations when it comes to special function amplifiers? Here, we’ll cover some of those variations and how to best use them in your designs.

Special function amplifiers can be programmable, but they don’t have to be. Instrumentation amplifiers (in amps or INA), programmable gain amplifiers (PGA), variable gain amplifiers (VGA), and digital variable gain amplifiers (DVGA) all fall in this category.

Traditionally, monolithic in amps  are designed using two and three op amp topologies along with high common mode rejection (thanks to well matched on-chip resistors), and high impedance usually at a lower cost than their discrete counterparts.

The two op amp option offers the advantage of using one fewer amplifier at the expense of a poorer common mode rejection due to the additional phase of the first op amp, or propagation delay. More conventional designs use a three op amp topology which provides better balancing and matching, as well as a higher common mode.

Regardless of their topology, in amps have one thing in common….differential inputs and single ended output with a reference pin that is connected to ground for dual supplies or biased to mid supply for a single supply operation. 

Other topologies are also used depending on the target application. Switched capacitors are used with chopper stabilized in amps for applications with low impedance sensors such as magnetic coils and strain gauges.  

Just like op amps, in amps can be designed with current mode or current feedback. The common mode is independent of resistor matching and the in amp can sense ground. The LMP8358 uses this technique, along with auto zeroing, to provide superior performance and on chip fault detection.

A major benefit of current mode topologies is the ground sensing capability. Rail to rail is also implemented by charge pumps at the expense of additional noise and an increase in die size.

PGAs differ from in amps in that they have a differential output. They don’t usually require an external gain setting resistor and in some cases they are used at gains lower than one. The attenuation feature is very useful in applications like structural testing a vibration control or universal IO modules for automation. These make great building blocks for integrated solution and they’re often part of high resolution analog to digital converter (ADC) as in the case of the ADS1220.

The latest PGA from the zero drift family is the PGA281. It has an input offset voltage of 5uV and a gain drift of 0.5ppm/ºC with EMI rejection of up to 120dB and a CMRR of 140dB! As for VGAs and DVGAs, they’re generally grouped into two types, constant output and constant input. In VGAs with constant output, the gain is adjusted to keep the output signal at a constant level while the input signal varies. In VGAs with constant input, the gain is adjusted to change the output signal while the input signal level is constant.

VGAs tend to be useful in applications requiring higher speed like sonar and photomultiplier control in industrial space, automatic gain control (AGC) in receivers for base stations, collision avoidance in automotive, video broadcasting  and ultrasound. And just like in amps, there are several topologies that can be implemented depending on the target applications.  The LMH6521 is just one of many DVGAs from TI that offer superior gain and phase matching.

The last type of special function amplifier is a complete analog front end (AFE) which combines gain and attenuation and provides users with the flexibility of single or differential inputs and outputs. The LMP7312 is an example of such AFE.

If you’re interested in learning more about special function amplifiers, specifically the basic difference between a current feedback amplifier (CFA) and a voltage feedback amplifier (VFA) check out my blog post called how to determine if a CFA or a VFA is better for your design.