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Hello, and welcome to the TI Precision Lab covering SAR ADC drive amplifier considerations when using fully differential amplifiers. Overall, this video will cover how to design data converter drive circuits using fully differential amplifiers, or FDAs. Specifically, in this section, we will learn the basic operation of an FDA. Also, we will cover how a single-ended signal can be converted to a fully differential signal using an FDA and an op amp.

Here we introduce the fully differential amplifier. An FDA is similar to a voltage feedback op amp with some key differences. Both the FDA and op amp have differential inputs, but the FDA has a differential output-- V out plus and V out minus. Furthermore, the FDA has an input pin, Vocm, that controls the common mode of the output signal. Also, the op amp has a single feedback path, where the FDA has two feedback paths. Since the FDA has two outputs, the dynamic range is twice as wide as an amplifier with the same power supplies.

Finally, the FDA has the benefit of canceling most even order harmonics, and thereby reducing distortion. Because many data converters have fully differential inputs, FDAs s are a natural choice as an input driver. Let's take a closer look at an FDA application example.

This is a basic FDA amplifier design. Notice the two feedback paths. The differential gain is calculated to be RFx divided by RGx which in this example is a gain of 1. The differential input is measured from the positive input to the negative input, which is 2 minus 1 volt, or 1 volt differential.

The output common mode voltage is set by the Vocm pin, which is 2.5 volts in this case. The differential output is equal to the differential input multiplied by the gain. So in this case, the output is 1 volt times a gain of 1. However, the output signals are shifted to force the common mode to match the Vocm pin setting. The voltage on the positive output is equal to the common mode voltage plus half the differential input voltage. The voltage on the negative output is equal to the common mode voltage minus half the differential input voltage.

Now let's look at an ADC circuit example. This slide shows some detail on the internal circuitry of a typical FDA, the THS4551. At the core of the device is an integrated differential high open loop gain forward path amplifier, as shown here. Also, integral to the design is a secondary error amplifier that controls the FDA's output common mode voltage. The air amplifier is typically a wide bandwith, low noise single-ended op amp that detects the common mode signal and generates an error signal that forces the output common mode voltage to be equal to Vocm.

The integrated 5.2 kiloohm resistors shown detect the feedforward amplifier's average output voltage, or in other words, it's common mode voltage. It then compares this average voltage against the DC voltage at it's non-inverting pin and adjusts its output voltage, which subsequently feeds into an internal bias node of the feedforward amplifier. Remember, an op amp in a negative feedback loop ideally has the same voltage at its inverting input and it's non-inverting input.

The negative feedback of the error amplifier therefore acts as a servo loop, adjusting the internal bias node of the feedforward amplifier until the voltage at its inverting input is equal to Vocm. When the Vocm pin is left floating, the voltage at the error amplifier's non-inverting input defaults to mid-supply because of the internal resistor divider between the amplifier supplies. If the Vocm pin is left floating, a capacitor should be connected externally from the Vocm pin to ground to reduce the high frequency thermal noise contribution from the 300 kiloohm resistors.

For applications that require an output common mode that is different from mid-supply, the circuit can drive the Vocm pin externally with a low output impedance DC source, usually a precision DAC or the VREF of an ADC. Note that for many SAR applications it is necessary to drive the Vocm pin because the common mode requirement may be tight.

This slide give some insight into the impact that using an FDA device has on distortion. Assume for a moment that the input signal has no or a minimal amount of distortion components. Any distortion introduced by the FDA is due to non-linearity in its transfer function. It's important to understand that these non-linearities are generally very small and for many applications may be negligible. Nevertheless, these non-linearities do exist and can be modeled as a polynomial.

At the top with a slide, we show the output function as a polynomial for the non-inverting output, and at the bottom of the slide, we show it for the inverting output. For these functions, x is a differentiable input signal. Notice that for the bottom signal, the polarity of the differential input is negative because it is inverted by the amplifier. The negative sign on the Vdif signal cancels out for even harmonics like x squared and x to the fourth power, whereas the negative sign is not affected for odd harmonics x cubed and x to the fifth power.

The differential output subtracts the inverting signal from the non-inverting signal. Assuming that the polynomial coefficients for both paths are equal, the even terms will cancel out and only odd harmonics will remain. So FDA devices will not generate even order harmonics as long as the inverting and non-inverting paths are properly balanced and have equivalent non-linearities.

To add to the discussion from the previous slide, any distortion components in the input signal will be directly passed to the output. The point here is that the FDA will not add to the even order harmonics, but any distortion components on the input will still be passed to the output. Sometimes engineers mistakenly believe that even order harmonics applied to the input of the FDA will be canceled by the FDA. But this is not the case.

Another important point is that the feedback resistors in the FDA must be well-matched in order to minimize distortion. Using 0.1% 20 PPM per degree C, metal film resistors will be sufficient for most low distortion designs.

Several precision data converters have a fully differential input. However, many applications produce single-ended signals. The single-ended signal can be converted to a fully differential amplifier with the circuit shown. In this example, the single-ended signal is unipolarity, meaning that the signal is always positive. In this example, the input is 0 volts to 5 volts, and the output is plus or minus 5 volts with a common mode of 2.5 volts. In general, the data converters input common mode requirement is equal to half the reference. A voltage divider connected to the reference voltage Vref is used to generate the common mode signal.

The high bandwidth buffer U3 prevents loading of the divider. It is important to use a wide bandwidth buffer because a switched capacitor SAR charge kickback will feed through the FDA's feedback network and into the amplifier's output. For the same reason, the input signal buffer U2 will need to be a wide bandwidth amplifier. The specific bandwidth requirement for the amplifier will be discussed later in detail. However, a quick rule of thumb is that the unity gain bandwidth should be greater than 5 divided by the acquisition time.

Some example signals are shown on the left hand side of the slide. The input is shown at the top in brown. Notice that the input is unipolar and ranges from 0 volts to 5 volts. The outputs are shown below in red and blue. The output range is plus or minus 5 volts. Notice that the two output signals are symmetrical about the common mode voltage, which is a constant 2.5 volts in this example.

The FDA Vocm voltage sets the common mode signal. The common mode control is critical because most fully differential SAR data converter specifications require the common mode to be at a constant voltage equal to half the reference voltage.

The input output relationship equations are given in the red box. These equations are used for an example input of 3.75 volts. Stepping through the math, the inverting and non-inverting output voltage is computed, as well as the differential output.

This circuit is almost identical to the one on the previous slide. The difference is that this one translates a bipolar, single-ended input to a fully differential signal, whereas the previous circuit converts a unipolar, single-ended signal to fully differential. From a hardware perspective, the only difference is the inverting input of the FDA. This circuit grounds are G1, whereas the previous circuit connected that resistor to half of the reference voltage.

Looking at the input signal shown in brown, you can see that this is a bipolar plus or minus 2 and 1/2 volt signal. The output signal is a plus or minus 5 volt fully differential signal with common mode set to half of the reference. Notice that the red signal on AIN P matches the input signal, and the blue signal is the inversion of the input signal. These two signals are above the common mode. The equations for this circuit are shown at the right, as well as an example for an input voltage of 3.75 volts.

A minor modification in the single-ended to differential circuits can improve the linear output swing. In this case, the only modification is to increase the common mode output by 0.1 volts. In this example, Vocm was increased from 2.5 volts to 2.6 volts. This will have the effect of shifting the output signals above ground by 0.1 volts to avoid any negative output swing limitations. It is important to realize that the input common mode specification only allows for a plus or minus 0.1 volt variation from the typical value. So you cannot shift the common mode more than 0.1 volts without violating the spec.

Another approach to improving the negative swing to the rail would be to use a small negative supply of negative 0.2 volts. The LM7795 charge pump is one way to generate this negative supply. The output swing for the positive supply can be improved by simply increasing the supply to 5.4 volts. The waveforms at the right show how the output signal is shifted above ground to avoid negative rail limitations.

For this example, the THS4521 FDA specifications are listed. The output swing low, or swing to the negative rail, is 0.15 volts worst case. So the 0.1 volt shift in this example doesn't allow for a full scale range linear output swing since the output only swings down to 0.1 volts. Nevertheless, it does improve the linear range by 0.1 volts. But the positive swing limitation, the supply is 5.4 volts. So the output swing is calculated by subtracting 0.3 volts from the supply, which is 5.1 volts. So the desired output swing is not limited for the positive rail.

That concludes this video. Thank you for watching. Please try the quiz to check your understanding of this video's content.

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