Download There are inherent problems in single-supply op

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 
2Rload Cout
2Rin Cin
2R1C1
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
2Rin C in
2R1C1
2Rload 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