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Download There are inherent problems in single-supply op
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IC provides temperature, bias and gain in single-supply applications By Reza Moghimi [[email protected]] Single-supply op-amp applications have inherent problems that are not usually encountered in dual-supply circuits. A reference voltage, usually at midpoint of the amplifier’s output range, must be established to allow a symmetrical output swing with respect to “common.” Typically accomplished by dividing the supply voltage with a pair of resistors, this apparently simple solution reduces stability and power supply rejection. Figure 1 illustrates the problem using an ac-coupled non-inverting amplifier. The in-band gain is G = 1 + R2/R1.The input is biased to Vs/2 by the R8/R9 divider pair. Capacitive coupling of the feedback through C1 introduces a zero, reduces the dc noise gain to unity, and keeps the dc level of output equal to the bias voltage. This prevents distortion caused by excessive amplification of the input offset voltage. The break frequencies at f=1/[2 R1C1] and f=1/[2 (R1+R2)C1], and those associated with the input- and output coupling circuits introduce phase shifts, thus increasing the possibility of oscillation. BW 1 1 BW 2 1 1 1 BW 4 BW 3 2Rload Cout 2Rin Cin 2R1C1 1 2 ( R8 )C 2 2 R Vout Vin(1 2 ) forX c1 R1 R1 Another potential limitation is the op amp's power supply rejection. In the absence of C2, changes in supply voltage will directly change the bias voltage set by the resistor divider. While this does not present a problem at dc, any common-mode noise appearing at the power-supply terminals will be amplified along with the input signal, except at the lowest frequencies. Adding C2 improves the power-supply rejection, but worsens the lowfrequency common-mode rejection, allowing substantial feedback through the power supply below 320 Hz. Larger capacitors are required to avoid “motor boating” and other stability problems. Even worse, unless the power supply is well bypassed and careful layout is used, a significant signal voltage will appear on the supply line when the op amp supplies a large output current into the load. With the non-inverting input referenced to the supply line, these signals will feed directly into the op amp, often in a phase relationship that will produce “motor boating” or other forms of oscillation. A more effective way to provide the bias in low-voltage, single-supply applications is to use an ADR821, which combines a precision low-power 2.5-V voltage reference and a unity-gain-stable op amp in a single package, as shown in Figure 2. BW 1 R 1 1 1 BW 2 BW 3 Vout Vin(1 2 ) forX c1 R1 2Rin C in 2R1C1 2Rload Cout R1 The benefits of this circuit are substantial. Using an ADR821 reduces the cost, power consumption, number of external components, and PC board area. The low-impedance reference improves the power-supply rejection and circuit stability. It can also be used as a reference for A/D and D/A converters. Featuring 0.2% initial accuracy and 15 ppm/°C temperature coefficient, the ADR821 consumes less than 400 μA, making it ideal for applications requiring both precision and low power. The ADR827 provides the same functionality, but substitutes a 1.25-V reference for the 2.5-V reference, making it useful in very low voltage applications. The temp pin of ADR827/821 can be used to monitor temperature of the system. The TC performance of this pin for few units is shown in figure below. TEMP pin is quite linear at approximately 1.9mV/C. TEMP OUT Vs TEMPERATURE 750 700 TEMP OUT - mV 650 600 550 500 450 400 -40 -20 0 20 40 60 TEMPERATURE - C 80 100 120