Improving Your RF Signal Chain With an RF Fully Differential Amplifier (FDA)

1 Understanding the Benefits Differential Signaling

Single-ended signaling is a simple and common way of transmitting an electrical signal from a transmitter to a receiver and vice versa. The single-ended electrical signal is transmitted by a voltage, which often varies and is referenced to a fixed potential, typically a 0-V node or ground; differential signaling employs two complementary inverted voltage signals referenced to a common-mode voltage to carry the information. Figure 1-1 demonstrates the conversion from a single-ended to differential conversion, where the differential signals are equal in magnitude, but opposite in polarity.

Figure 1-1 Unbalanced-to-Balanced Signal Through a Theoretical Passive Balun

Differential configuration allows the common-mode noise or crosstalk to come in as a common-mode signal that is equal in both lines and subtracted at the receiver. This allows differential signals to be much more robust due to a differential topology having inherent common-mode noise immunity. A differential receiver extracts information by detecting the difference between the inverted and non-inverted signals. The two voltage signals are balanced, meaning that they have equal amplitude and opposite polarity relative to a common-mode voltage. The return currents associated with these voltages are also balanced and thus cancel each other out.

Common-mode rejection ratio (CMRR) is often specified for fully differential ADC inputs and describes the ADCs ability to reject a common-mode (typically DC) voltage. A DC common-mode voltage appearing across the ADC inputs has the same effect as a DC input offset. Normally, if the signal and ground are in close proximity physically and will couple in common-mode noise. CMRR is defined as the ratio of differential voltage gain and the common-mode voltage gain:

Equation 1.

C M R R = \frac{D i f f e r e n t i a l V o l t a g e G a i n}{C o m m o n - M o d e V o l t a g e G a i n}

Note that CMRR is a frequency-dependent parameter. As the frequency of the common-mode voltage increases, the phase matching between the non-inverted and inverted signal for optimal common-mode rejection becomes more difficult to sustain. As a result, good common-mode rejection is harder to obtain at higher frequencies.

One of the key advantages of differential signals is the increased dynamic range. With power supplies dropping to 3.3 V and lower for high-speed and RF data converters, design engineers are looking for ways to achieve greater input dynamic range. In theory, given the same voltage range for single-ended and fully differential inputs, the fully differential inputs will have twice the dynamic range. This is because the two differential inputs can be 180° out of phase, as shown in Figure 1-1. Another way to think about this advantage is the relationship to signal-to-noise ratio (SNR). The SNR is defined in terms of the full-scale input level and the minimum detectable signal of the ADC:

Equation 2.

M A X S N R = 20 \log (\frac{F u l l S c a l e V o l t a g e L e v e l}{M i n i m u m D e t e c t a b l e S i g n a l})

Typically, the minimum detectable signal is limited by the noise floor. Since fully differential inputs have twice the full-scale input voltage level with approximately the same level of noise as its single-ended configuration, while having superior DC and AC common-mode rejection, SNR increases. However, many single-ended signals must maintain a relatively high voltage to ensure adequate SNR. Common single-ended interface voltages are 5 V or higher. Because of the improved immunity to noise, the differential approach in lower voltage systems is advantageous to maintain adequate SNR.