# Single-ended-to-differential converter has resistor-programmable gain

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Many applications—such as driving modern ADCs, transmitting signals over twisted-pair cables, and conditioning high-fidelity audio signals—require differential signaling to achieve higher signal-to-noise ratios, increased common-mode noise immunity, and lower second-harmonic distortion. This requirement presents a need for a circuit block that can convert single-ended signals to differential signals; that is, a single-ended-to-differential converter.

For many applications, an AD8476
precision, low-power, fully differential
amplifier with integrated precision resistors
is more than adequate to perform the
single-ended-to-differential conversion
function. For applications that require
improved performance, however, an
OP1177 precision op amp can be cascaded
with the AD8476, as shown in
**Figure 1**. This single-ended-to-differential
converter has high input impedance;
2-nA (max) input bias current;
60-μV (max) offset voltage, referred to
the input; and 0.7-μV/°C
(max) offset voltage drift,
referred to the input.

**Figure 1** You set the gain of this single-ended-to-differential converter by adjusting the ratio of R_{F} to R_{G}.

The presented circuit is a two-amplifier feedback arrangement in which the op amp determines the circuit’s precision and noise performance, while the differential amplifier performs the single-ended-to-differential conversion. This feedback arrangement suppresses the errors of the AD8476, including noise, distortion, offset, and offset drift, by placing the AD8476 inside the op amp’s feedback loop, with the op amp’s large open-loop gain preceding it. In essence, the arrangement attenuates the errors of the AD8476 by the open-loop gain of the op amp when referred to the input.

External resistors R_{F} and R_{G} set the
gain of the single-ended-to-differential
converter in **Figure 1** such that

A minimum gain of two can be achieved
by replacing R_{F} with a short and R_{G} with
an open.

As with any feedback connection,
care must be taken to ensure the system
is stable. The cascade of the OP1177
and the AD8476 forms a composite
differential-out op amp whose open-loop
gain over frequency is the product
of the OP1177’s open-loop gain and
the AD8476’s closed-loop gain. The
closed-loop bandwidth of the AD8476,
therefore, adds a pole to the open-loop
gain of the OP1177. To ensure stability,
the bandwidth of the AD8476 should
be higher than the unity-gain frequency
of the OP1177. This requirement is
relaxed when the circuit is in a closed-loop
gain greater than two, because the resistor feedback network effectively
reduces the unity-gain frequency of the
OP1177 by a factor of R_{G}/(R_{G}+R_{F}). The
AD8476 has a bandwidth of 5 MHz, and
the OP1177 has a unity-gain frequency
of 1 MHz, so the circuit shown does not
exhibit stability issues at any gain.

When using an op amp with a unity-gain
frequency that is much larger than
the differential amplifier’s bandwidth,
you can insert a bandwidth-limiting
capacitor, C_{F}, as shown in **Figure 1**.
Capacitor C_{F} forms an integrator with
the feedback resistor R_{F} such that the
bandwidth of the overall circuit is
given by

The factor of one-half in the bandwidth equation is due to the circuit’s output being fed back single-endedly rather than differentially. As a result, the circuit’s feedback factor and bandwidth are reduced by two.

If this reduced bandwidth is lower
than the closed-loop bandwidth of the
differential amplifier, the circuit will be
stable. This bandwidth-limiting technique
also can be employed with a gain
of two by making R_{G} an open circuit.

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