SBAA634 April 2026 TAA5212 , TAA5412-Q1 , TAC5111 , TAC5111-Q1 , TAC5112 , TAC5112-Q1 , TAC5211 , TAC5212 , TAC5311-Q1 , TAC5312-Q1 , TAC5412-Q1 , TAD5112 , TAD5112-Q1 , TAD5212 , TAD5212-Q1

1 Introduction

Figure 1-1 (a) describes a differential-input amplifier, with input signals V_IN+ and V_IN- on the (+) and (-) terminals respectively.

Figure 1-1 Differential-Input Amplifier

For an ideal amplifier, the output V_OUT is:

Equation 1.

V_{O U T} = A_{d} * (V_{I N +} + V_{I N -})

Where A_d is the differential gain of the amplifier.

However, in a practical amplifier, mismatches between the (+) and (-) signals paths can result in an additional component to the output signal. To compute this, the circuit in Fig 1-1 (b) can be equivalently represented as shown in Figure 1-1 (b), with a "differential signal" V_DIFF, and a "common-mode" signal "V_CM". For such an amplifier, the output is given by:

Equation 2.

V_{O U T} = A_{d} * V_{D I F F} + A_{c} * V_{C M}

Where A_d is the differential gain, and A_c is called the common-mode gain of the amplifier. These signals are given by:

Equation 3.

V_{D I F F} = V_{I N +} - V_{I N -}

Equation 4.

V_{C M} = \frac{V_{I N +} + V_{I N -}}{2}

One of the key performance parameters for a differential input system is it's ability to "reject" such common-mode signals. This is called "Common-Mode Rejection Ratio (CMRR)", and the value of CMRR in dB is given by:

Equation 5.

C M R R (d B) = 20 * l o g (\frac{1}{A_{c}})

For an ideal differential amplifier, the CMRR is ∞ dB.

The CMRR of a non-ideal amplifier can be measured by providing the same signal V_CM at both the input terminals and observing the output V_OUT. The CMRR can then be calculated as:

Equation 6.

C M R R (d B) = 20 * \log (\frac{V_{C M}}{V_{O U T}})

However, such circuits also have a specified range of common-mode swing which they can tolerate. In this document, the common-mode swing tolerance for the TAC5x1x/TAC5x1x-Q1 devices is discussed, along with how this tolerance can be programmed, and its impact on device performance.