Figure 1: A non-functional, AC-coupled op amp circuit

AC coupling is an easy way to block
dc voltages that are present at the in amp's inputs. This is especially
useful in high-gain applications, where even a small DC voltage
at an amplifier's input can limit its dynamic range. But AC coupling
into a high impedance input, without providing a dc return, renders the
circuit nonfunctional!

What actually happens is the input
bias currents will charge up the AC coupling capacitors until the input
common-mode voltage is exceeded. In other words, the caps will charge
up to the positive supply line, or down to the negative supply,
depending on the direction of the input bias currents.

Of special concern is that it could
take several minutes for the bias currents to charge up a FET-input
device when using very large AC input-coupling capacitors, so there may
be a long delay before the amplifier is rendered inoperative. As a
result, a casual lab test might not detect this problem, leading to
circuit failure in the field. Therefore, it's very important to avoid
this problem altogether.

Figure 2shows a simple solution to this very common problem.

Figure 2: Correct method for AC coupling an op-amp input for dual-supply operation

In this circuit, a resistor is
connected between the op-amp input and ground, to allow the input bias
currents to flow. For lowest input-offset currents using bipolar op
amps, R₁ is usually set equal to the parallel combination of R₂ and R₃.

Note, however, that this resistor will always introduce some noise into the circuit, so there will be a tradeoff between circuit
input impedance, the size of the input-coupling capacitor needed, and
the Johnson noise added by the resistor. Practical resistor values vary
from around 100 kΩ to 1 MΩ.

A similar problem affects instrumentation amplifiers. Figure 3a and Figure 3b
show in-amp circuits which are AC coupled using two capacitors but
which, again, do not provide an input bias-current return path. This
problem is common with both single and dual power-supply in-amp
circuits.

Figure 3: Examples of non-functional, AC-coupled in-amp circuits

Figure 4 shows a
transformer-coupled circuit. This circuit is non-functional since it,
too, does not provide a DC return path to ground.

Figure 4: A non-functional, transformer-coupled, in-amp circuit

Simple solutions for these circuits are shown in Figure 5 and Figure 6. Here, a high-value resistance (R_a, R_b) is added between each input and ground. This is a simple and practical solution for dual-supply in-amp circuits.

Figure 5: A high-value resistor between each input and ground supplies the necessary bias-current return path

The resistors allow a discharge
path for input bias currents. In the dual-supply example of Figure 5a,
both inputs are now referenced to ground. In the single-supply example
of Figure 5b, the inputs may either be ground referenced (V_cm tied to ground) or connected to a bias voltage equal to the midpoint of the maximum input-voltage range.

Figure 6 shows a typical solution
for transformer coupled inputs. In these circuits, there will be a
small offset-voltage error due to the mismatch between the input bias
currents flowing through the two non-identical resistors. To avoid
errors due to an R_a/R_b mismatch, a third
resistor, of about one-tenth their value, can be connected between the
two in-amp inputs (thus bridging both resistors).

Figure 6: Correct method for transformer input coupling to an in amp

Note that, when using transformers
with a center-tapped secondary winding, this can often be grounded
directly, thus avoiding the need for the two resistors.

2) Supplying reference voltages for in amps, op amps, and ADCs

Figure 7 shows a single-supply circuit where an in amp is
driving an analog/digital converter (ADC). Many such circuits use
resistor dividers or other simple methods to supply the in amp and ADC
reference voltages.

Note that the two reference voltages may be quite different. As shown, a simple RC low-pass filter is often used between in-amp output and ADC input to reduce noise.

Figure 7: A typical single-supply circuit where an in-amp is driving an ADC

(Click on figure to enlarge)

3) Driving in-amp reference inputs

A common application problem occurs when designers try to connect
high-impedance sources to the reference pins of in amps. This often
introduces serious errors, especially in in-amp circuits which are
comprised of three op amps, and their monolithic IC equivalents. Figure 8 makes this problem easier to understand.

Figure 8. Incorrect use of a simple voltage divider to directly drive the reference pin of an in amp

(Click on figure to enlarge)

This example uses the standard
three op-amp in-amp circuit; most IC in-amps also follow this same
basic architecture. The in amp has its reference pin tied directly to a
simple voltage divider. The voltage divider ties directly to resistor R₄ in the in-amp's subtractor section; therefore, it becomes part of the R₃/R₄ subtractor network. Note that any added resistance between R₄ and ground increases the total resistance of R4 and unbalances the subtractor section.

This, in turn, reduces both the
in-amp's common-mode rejection and its gain accuracy. Attempts to use
small resistor values in the voltage divider will increase power-supply
current consumption, may overheat the resistors, and certainly is not a
good design approach.

Figure 9 shows a much better
solution: here an op-amp buffer amplifier is inserted between the
voltage divider and the in-amp's reference pin.

Figure 9: Driving the reference pin of an in amp from the low-impedance
output of an op amp

(Click on figure to enlarge)

However, note that even though the
reference is now being driven by a very low-impedance source (typically
less than one ohm), the addition of the op-amp buffer alone still
leaves a potential problem: lack of adequate power-supply rejection.

4) Preserving PSRR when amplifiers are referenced from the supply line using voltage dividers.

Often overlooked is the very important issue of power-supply rejection (PSR) both with in-amp and op-amp circuits. Power supply
rejection isolates an amplifier from power-supply hum, noise, and any
voltage variations present on the power-supply line. This is important
because many real-world circuits are powered by less than ideal
supplies. Also, any AC signals present on the supply lines can be fed
back into the circuit, amplified and, if conditions are right, cause a
parasitic oscillation.

Modern op amps and in amps all
provide substantial power-supply rejection as part of their design.
This is something that most engineers take for granted. Many modern op
amps and in amps have power-supply rejection ratio (PSRR)
specifications of 80 dB to over 100 dB, reducing power-supply
variations on the amplifier by a factor of 10,000 to 100,000. Even a
fairly modest value of 40 dB PSRR isolates the supply from the
amplifier by a factor of 100.

However, when designers use a
simple resistor divider and op-amp buffer to supply a reference voltage
for an in amp, any variations in power-supply voltage travel through
this circuitry and directly vary the in-amp's output level. Therefore,
unless low-pass filtering is provided, the normally excellent PSRR of
the IC is lost.

Figure 10: Decoupling the reference circuit to preserve the in-amp PSRR

(Click on figure to enlarge)

In Figure 10, a large
capacitor has been added to the voltage-divider network to decouple it
from power-supply variations and preserve PSRR.

Here the -3 dB pole of this filter is set by the parallel combination of R₁/R₂ and the decoupling capacitor C₁. The pole should be set approximately ten-times lower than the low-frequency bandwidth of the circuit. The -3 dB pole frequency is equal to 1/(2πRC₁), where R= (R₁ • R₂)/2).

A good "cookbook" rule of thumb is to use two 100 kΩ resistors for R₁ and R₂, and to make C₁
100 μF or more. This provides a -3 dB pole frequency of approximately
0.03 Hz. Note that the second small capacitor across the 50 kΩ op-amp
feedback resistor simply cancels-out any resistor noise.

Designers should also keep in mind that the R₁/R₂, C₁
network will take time to charge up. Using our cookbook values, that
turn-on time T is equal to RC, where R equals 50 kΩs (the parallel
combination of R₁, R₂) and C₁ equals 100 μF. In this case, the voltage applied to the in-amp's reference pin will be delayed by five seconds.

The circuit of Figure 11 offers a further refinement.

Figure 11: An op-amp buffer, operating as an active filter, drives the reference
pin of an in-amp

(Click on figure to enlarge)

Here, the op-amp buffer is operated
as an active filter, which allows the use of much-smaller capacitors
for the same amount of power-supply decoupling. In addition, the active
filter can be designed to provide a higher Q and thus give a quicker
turn-on time.

Test results: With the component
values shown, and +12 V applied, a +6 V filtered reference voltage was
provided to the in amp. A 1 V_p-p sinewave of varying frequency was used to modulate the +12 V supply, with the in-amp gain set to one.

Under these conditions, no AC signal was visible on an oscilloscope, at V_ref
or at the in-amp output, until approximately 8 Hz. Measured supply
range for this circuit was +4 V to greater than +25 V, with a low-level
input signal applied to the in amp. Circuit turn-on time was
approximately two seconds.

5) Decoupling single-supply op-amp circuits

Single-supply op-amp circuits are often referenced from the
power-supply line using voltage dividers. Therefore, they also require
adequate decoupling to preserve PSRR.

In single-supply circuits, a very
common and incorrect practice is to use a 100 kΩ/100 kΩ resistor
divider with a 0.1 μF bypass capacitor, to supply V_s/2 to
the inverting pin of the op amp. Using these values, power-supply
decoupling is often inadequate, as the pole frequency is only 300 Hz.
Circuit instability ("motor boating") often occurs, especially when
driving inductive loads.

Figure 12 and Figure 13 show examples for inverting and non-inverting single-supply op-amp circuits.

Figure 12: A single-supply non-inverting amplifier circuit showing correct power-supply decoupling

(Click on figure to enlarge)

Figure 13: Proper decoupling for a single-supply inverting amplifier circuit

(Click on figure to enlarge)

A good rule of thumb when using a 100 kΩ/100 kΩ voltage divider, as shown, is to use a C₂
value of at least 10 μF for a 0.3 Hz, -3 dB rolloff. A value of 100 μF
(0.03 Hz pole) should be sufficient for nearly all circuits.