Amplifier circuit offsets are always an important topic for any amplifier design. Most voltage amplifier circuit offsets are a result of the amplifier’s input voltage offset, input bias current and input offset currents. These values, in combination with the values of the circuit’s passive elements and any input source offset, combine for a total offset voltage measured at the output of the amplifier. For more information on offsets in standard operational amplifiers (op amps), see the TI Precision Labs section on input offset voltage and bias currents.
Fully differential amplifiers (FDAs) have all of the same offsets as a normal operational amplifier, but these offsets have components relative to both the differential- and common-mode portions of the amplifier outputs. FDAs also contain another offset component between the common-mode pin and the amplifier’s output common-mode voltage. When observing only the amplifier’s total output offset, it can quickly become challenging to understand the primary source of the offset, and therefore challenging to properly compensate for the various sources. The best way to identify and compensate for the sources of offsets in FDAs is to understand the effect of each individual offset source, and then apply that knowledge to decipher the combined output offset of a specific circuit.
Differential- versus common-mode offsets
Before looking at any sources of offsets, it is first important to understand how offsets will manifest at the output of an FDA. Because FDAs have differential outputs, an offset can appear as either a differential- or a common-mode offset at the output. The differential signal is defined as the difference between the two FDA outputs and the common-mode signal is the average of those two outputs. Figure 1 illustrates the two effects and defines the two signal types.
Always observe both the differential- and common-mode output signals to diagnose offset effects. The effects of the differential signal will not show any changes in the common-mode signal; conversely, the common-mode signal will not show any changes in the differential signal. With a solid understanding of these two offset types, it is then possible to analyze how each offset source will result in an output offset.
Input voltage offsets
The easiest source of offset to understand in differential amplifiers is the input voltage offset. For FDAs, the input offset is typically specified in the data sheet, very similarly to a single-ended operational amplifier. Figure 2 is an excerpt from the Texas Instruments (TI) THS4551 FDA data sheet electrical characteristics table, detailing the input offset voltage and corresponding drift performance for this device.
The voltage offset can be modeled as a small voltage source in series with one of the amplifier’s inputs. Because the input voltage offset appears as a voltage between the two inputs, it only manifests as a differential signal at the inputs of the device. Figure 3 shows an example of how to model the input offset voltage of an FDA using an external voltage source.
Offset voltage model for an FDA
Because the input offset voltage appears as a differential source, it will only appear as a differential signal at the device output with a corresponding gain of 1 + RF/RG, similar to a standard op amp. Using the example of the THS4551 amplifier offset in Figure 2, a circuit with RF = RG could expect a typical output offset due to the input offset voltage of +/-100 μV.
Common-mode loop offset
A unique offset component that is only present in FDAs is the offset between the output common mode and the set common-mode value. One of the unique features of FDAs is the presence of a common-mode feedback loop that adjusts the output common-mode voltage to a set value. Most devices will feature an output common-mode pin that sets the desired voltage. There is often an offset voltage between the set value and the actual measured output common-mode voltage, however.
Again using the THS4551 as an example, Figure 4 shows the output common-mode pin offset specifications. Because the output common-mode control loop has only a gain of 1 V/V, the common-mode offset at the output would typically be 1 mV. Fortunately, it’s possible to almost entirely eliminate this error by using a precision error-correction loop such as the one shown in Figure 72 of TI’s LMH5401 data sheet.
Offsets due to currents
A slightly less intuitive offset component in FDAs is the effect due to the input bias currents flowing into or out of each input. These currents are usually specified two ways: as the amount of current flowing the same direction into or out of each input (specified as “input bias current”) and as the difference of current flowing between the inputs (specified as “input offset current”).
In a single-ended op amp, the input bias current can manifest as a voltage offset at the output based on the resistor values used in the feedback loop. However, for FDAs, the effect of the bias current is actually cancelled by the common-mode feedback loop and will not have an effect on the output. Essentially, the bias currents induce a common-mode offset at the output, but the common-mode loop detects this error and adjusts the amplifier’s common mode accordingly.
Unfortunately, input offset current still has an effect on the total output offset of an FDA. The offset current is most easily modeled as a current source attached to each amplifier input – one sinking half of the offset current, and the other sourcing half of the offset current. Figure 5 shows an example of an offset current model for an FDA.
Input offset current model for an FDA
Each half of the offset currents will flow through the corresponding feedback resistor and impart a differential offset voltage at the output. The resulting total differential output offset from the input offset current is equal to ios × RF.
Figure 6 shows the input offset current specifications for the THS4551 FDA used in earlier examples. It is easy to see that the input offset current of +/-10 nA typical is over an order of magnitude smaller than the offset voltage specification of +/-50 μV shown in Figure 2. However, because the offset current effect is multiplied by the absolute value of the feedback resistor, it can become a significant factor of total output offset in the presence of large feedback resistors.
Offsets from sources
Perhaps the most confusing source of offset errors for FDAs is not actually from the amplifier itself, but from the source driving it. For differential sources it is fairly straightforward: a differential offset of the source causes a differential offset at the output; any common-mode offset of the source is cancelled by the output common-mode loop of the amplifier. However, with single-ended to differential circuits (one of the most common uses of FDAs), the offset translation becomes much less intuitive.
When using an FDA in a single-ended to differential signal circuit configuration, it is certainly possible that the single-ended source may have a DC offset component in the signal. In the case of single-ended operational amplifiers, this DC source offset may sometimes be referred to as a “common-mode offset,” which confusingly, is different than the definition of common mode for differential signals. In the case of a single-ended input source with a DC or “common-mode” offset, that offset will actually translate to a differential offset at the output of an FDA. This differential offset is caused because the DC offset of the input signal actually appears as a constant differential voltage between the amplifier’s signal input and non-signal input. Figure 7 illustrates this concept and the resulting potential for a large differential offset error at the output.
The effect of a differential output offset due to the offset of a single-ended input signal can often be one the largest sources of confusion when analyzing offset errors in fully differential circuits. Fortunately, it’s possible to cancel this effect by applying an equivalent DC signal to the non-input side of the fully differential circuit to balance the DC component of the two amplifier inputs.
Importance of minimizing offsets
With FDAs, it is important to minimize output errors as much as possible in a given circuit. For example, FDAs are often used to drive high-resolution analog-to-digital converters (ADCs), where each offset error results in a loss of total output swing and therefore less signal swing into the ADC. It is possible to minimize many of the offset components present in FDA circuits if you have a solid understanding of how each portion of the circuit contributes to the total output offset, thus maximizing the total performance of FDA applications.