A Differential Op-Amp Circuit Collection

All op-amps are differential input devices. Designers are accustomed to working with

these inputs and connecting each to the proper potential. What happens when there are

two outputs? How does a designer connect the second output? How are gain stages

and filters developed? This application note will answer these questions and give a

jumpstart to apprehensive designers.

The idea of fully-differential op-amps is not new. The first commercial op-amp, the K2-W,

utilized two dual section tubes (4 active circuit elements) to implement an op-amp with

differential inputs and outputs. It required a ±300 Vdc power supply, dissipating 4.5 W of power,

had a corner frequency of 1 Hz, and a gain bandwidth product of 1 MHz(1).

In an era of discrete tube or transistor op-amp modules, any potential advantage to be gained

from fully-differential circuitry was masked by primitive op-amp module performance. Fullydifferential

output op-amps were abandoned in favor of single ended op-amps. Fully-differential

op-amps were all but forgotten, even when IC technology was developed. The main reason

appears to be the simplicity of using single ended op-amps. The number of passive components

required to support a fully-differential circuit is approximately double that of a single-ended

circuit. The thinking may have been "Why double the number of passive components when

there is nothing to be gained?"

Almost 50 years later, IC processing has matured to the point that fully-differential op-amps are

possible that offer significant advantage over their single-ended cousins. The advantages of

differential logic have been exploited for 2 decades. More recently, advanced high-speed A/D

converters have adopted differential inputs. Single-ended op-amps require a problematic

transformer to interface to these differential input A/D converters. This is the application that

spurred the development of fully-differential op-amps. An op-amp with differential outputs,

however, has far more uses than one application.

BASIC CIRCUITS

The easiest way to construct fully-differential circuits is to think of the inverting op-amp feedback

topology. In fully-differential op-amp circuits, there are two inverting feedback paths:

. Inverting input to noninverting output

. Noninverting input to inverting output

Both feedback paths must be closed in order for the fully-differential op-amp to operate properly.

When a gain is specified in the following sections, it is a differential gain - that is the gain at

VOUT+ with a return of VOUT-. Another way of thinking of differential outputs is that each signal is

the return path for the other.

A New Pin

Fully-differential op-amps have an extra input pin (VOCM). The purpose of this pin is to provide a

place to input a potentially noisy signal that will appear simultaneously on both inputs - i.e.

common mode noise. The fully-differential op-amp can then reject the common mode noise.

The VOCM pin can be connected to a data converter reference voltage pin to achieve tight tracking

between the op-amp common mode voltage and the data converter common mode voltage. In

this application, the data converter also provides a free dc level conversion for single supply

circuits. The common mode voltage of the data converter is also the dc operating point of the

single-supply circuit. The designer should take care, however, that the dc operating point of the

circuit is within the common mode range of the op-amp + and - inputs. This can most easily be

achieved by summing a dc level into the inputs equal or close to the common mode voltage.

Gain

A gain stage is a basic op-amp circuit. Nothing has really changed from the single-ended

design, except that two feedback pathways have been closed. The differential gain is still Rf /Rin

a familiar concept to analog designers.

Gain = Rf/Rin

This circuit can be converted to a single-ended input by connecting either of the signal inputs to

ground. The gain equation remains unchanged, because the gain is the differential gain.

Instrumentation

An instrumentation amplifier can be constructed from two single-ended amplifiers and a fullydifferential

amplifier as shown in Figure 2. Both polarities of the output signal are available, of

course, and there is no ground dependence.

FILTER CIRCUITS

Filtering is done to eliminate unwanted content in audio, among other things. Differential filters

that do the same job to differential signals as their single-ended cousins do to single-ended

signals can be applied.

For differential filter implementations, the components are simply mirror imaged for each

feedback loop. The components in the top feedback loop are designated A, and those in the

bottom feedback loop are designated B.

For clarity decoupling components are not shown in the following schematics. Proper operation

of high-speed op-amps requires proper decoupling techniques. That does not mean a shotgun

approach of using inexpensive 0.1-μF capacitors. Decoupling component selection should be

based on the frequencies that need to be rejected, and the characteristics of the capacitors used

at those frequencies.

Single Pole Filters

Single pole filters are the simplest filters to implement with single-ended op-amps, and the same

holds true with fully-differential amplifiers.

Double Pole Filters

Many double pole filter topologies incorporate positive and negative feedback, and therefore

have no differential implementation. Others employ only negative feedback, but use the

noninverting input for signal input, and also have no differential implementation. This limits the

number of options for designers, because both feedback paths must return to an input.

The good news, however, is that there are topologies available to form differential low pass, high

pass, bandpass, and notch filters. However, the designer might have to use an unfamiliar

topology or more op-amps than would have been required for a single-ended circuit.

Multiple Feedback Filters

MFB filter topology is the simplest topology that will support fully-differential filters.

Unfortunately, the MFB topology is a bit hard to work with, but component ratios are shown for

common unity gain filters.

Akerberg Mossberg Filter

Akerberg Mossberg filter topology is a double pole topology that is available in low pass, high

pass, band pass, and notch. The single ended implementation of this filter topology has an

additional op-amp to invert the output of the first op-amp. That inversion in inherent in the fullydifferential

op-amp, and therefore is taken directly off the first stage.

Biquad Filter

Biquad filter topology is a double pole topology that is available in low pass, high pass, band

pass, and notch. The highpass and notch versions, however, require additional op-amps, and

therefore this topology is not optimum for them. The single-ended implementation of this filter

topology has an additional op-amp to invert the output of the first op-amp. That inversion is

inherent in the fully-differential op-amp, and therefore is taken directly off the first stage.

Danny Camperos CRF

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## domingo, 21 de marzo de 2010

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