Analog Devices AN-402 Application Notes
AN-402
a
ONE TECHNOLOGY WAY • P.O. BOX 9106
Replacing Output Clamping Op Amps with Input Clamping Amps
INTRODUCTION
Various systems like ultrasound and imaging systems,
have instances where the analog signal might suddenly
spike to a voltage extreme. But many downstream circuits like A/D drivers place restrictions on the analog input signal levels in order to maintain their performance.
These devices can draw excessive current in an overdrive condition or else be driven into a region of saturation which will have a long recovery time.
Various clamp amps can be used in these systems to restrict signal excursions at their outputs to protect downstream devices. So far most of the clamping amplifiers
have relied upon an output clamping architecture and
are called output clamp amps (OCAs). A new architecture called an input clamp amp (ICA) offers superior
clamping accuracy and lower distortion.
Figure 1 illustrates the relative performance of the two
devices. It can be seen that the ICA more closely tracks a
straight line in the linear region up to the point that it
bends over into the clamp region. On the other hand, the
OCA breaks away from a straight line sooner as it approaches the clamp voltage. Of course the extent to
which the response more closely tracks a straight line is
indicative of the amplifier’s linearity in that region.
1.6
CLAMP REGION
1.4
OUT
1.2
1.0
AD8036
OUTPUT VOLTAGE – V
0.8
0.6
0.6 2.00.8 1.0 1.2 1.4 1.6 1.8
Figure 1. Output Clamp Error vs. Input Clamp Error
LINEAR
EXTENSION
CLAMP
ERROR – 25mV
AD8036
(ICA)
OUTPUT CLAMP AMP
LINEAR REGION
CLAMP ERROR – >200mV
OUTPUT CLAMP
NEAR CLAMP REGION
INPUT VOLTAGE – +V
IN
CLAMP
LEVEL
•
NORWOOD, MASSACHUSETTS 02062-9106
APPLICATION NOTE
617/329-4700
•
To compensate for this added distortion, OCAs are required to have their clamping levels set wider than the
maximum excursion of interest if minimum distortion is
desired. Therefore, when substituting an ICA for an
OCA, the clamp region can be narrowed without adding
extra distortion. This will lower the voltages experienced by downstream circuitry during overdrive. In
most designs, making this adjustment requires only a
minor modification to the circuit that generates the
clamp voltages.
Figure 2 illustrates this concept. The amplitude of the
linear signal for each type of amplifier is the same. However, the upper and lower clamp levels of the OCA must
be set wider in order to preserve the signal linearity due
to the larger near-clamp distortion region. Thus for overdrive conditions, the downstream circuitry will see
larger signals when driven by an OCA than an ICA.
NEAR CLAMP
DISTORTION
INPUT
CLAMP
AMP
OUTPUT
CLAMP
AMP
REGION
NEAR CLAMP
DISTORTION
REGION
Figure 2. Comparison of Near-Clamp Distortion
Regions of ICA vs. OCA
In addition the overdrive response will be further improved as a result of the superior overdrive characteristics of the ICA. The output of an ICA will not go more
than 10 mV past the level set by the clamps for clamping
stages with low gain. On the other hand, an OCA will
HIGH CLAMP LEVEL
MAX
LINEAR
SIGNAL
REGION
LOW CLAMP LEVEL
HIGH CLAMP LEVEL
MAX
LINEAR
SIGNAL
LOW CLAMP LEVEL
overshoot by a few hundred millivolts depending on the
magnitude of the overdrive signal. Once again Figure 1
illustrates this concept. The ICA performance can be
seen to be relatively flat in the clamp region independent of the magnitude of the overdrive, while the OCA
output keeps on increasing along with increasing overdrive amplitude.
NONINVERTING OPERATION
Unity Gain
For the case of substituting for a noninverting OCA, the
most important consideration is the gain at which the
clamp amp is operating. This is because the output
clamping level for an ICA is a function of the closed loop
gain of the amplifier.
The first two input clamp amps, the AD8036 and
AD8037, introduced by Analog Devices operate with an
ICA structure. But because of differences in their operation, except for circuits that operate with a gain of +1,
substituting an ICA into a design that has been implemented with an OCA is not a “drop-in” replacement,
even though the pinouts of the parts are identical. However, because the pinouts are identical, the required circuit modifications will, in general, be not too extensive.
Each configuration though must be handled on a caseby-case basis. The following details the considerations
for making this substitution.
Inverting Operation
The first consideration is the polarity of operation of the
op amp. The input clamping op amp architecture of the
AD8036 and AD8037 does not operate in the inverting
mode. Therefore it is not possible to directly replace an
OCA with an ICA for inverting configurations. In order to
benefit from the ICA’s superior clamping characteristics
in inverting applications, a separate inverting stage is
required.
Figure 3 shows a circuit with an inverting stage followed
by an ICA, the AD8036 in a noninverting configuration
for providing the overall function of an inverting clamping amplifier. The circuit shown will have a gain of
–R
and will clamp at VH and VL. The operation of the
F/RI
clamping stage will be explained further in the next section. In all clamp circuits, V
must be greater than VL, but
H
the two can be anywhere within the output range of the
part.
140Ω
V
R
F
R
I
V
IN
2
6
3
H
2
8
V
6
H
3
V
L
AD8036
5
V
L
V
OUT
Figure 3. Inverting Clamping Circuit
For circuits that require a gain of more than (minus) 1,
the designer has a choice as to how to distribute the
gain between the inverting stage and the clamp stage.
For greatest accuracy, the ICA should operate at lower
gains because the clamp accuracy is a function of the
gain as will be detailed in the next section. Additional
required gain can be provided in the inverting stage.
The first case to consider is a noninverting unity gain.
For OCAs, the clamping levels are simply equal to the
voltages applied to V
(Pin 8) and VL (Pin 5). For an ICA,
H
these voltages are multiplied by the closed loop gain in
order to calculate the clamping levels. But since the gain
is +1, the ICA and OCA will both have the same clamping
levels. Thus, a direct substitution can be made. Figure 4
is an example of a unity gain clamping circuit.
V
CH
0.1µF
100Ω
V
IN
3
2
0.1µF
8
V
H
AD8036
V
L
5
300Ω
V
CL
+5V
0.1µF
7
4
0.1µF
R
F
–5V
10µF
6
10µF
V
OUT
Figure 4. Unity Gain Noninverting Clamp
Since we are talking about a noninverting unity gain, the
amplifier chosen must also exhibit stable operation at
unity gain. Of the two ICAs, the AD8036 is compensated
for operation at unity gain. Thus, the AD8036 is a “drop
in” replacement for an OCA in noninverting unity gain
applications. It will provide the same gain and clamp at
the same levels as the OCA.
Gains of Two or More
When the noninverting gain of the clamp amp is two or
greater, the AD8037 can be used for its wider bandwidth, as it is compensated for noise gains of two or
greater. However, the voltages applied to the clamp pins
will have to be changed to maintain the same clamping
levels, because the clamping levels are a function of the
closed loop gain of the amplifier. The following equations summarize the calculations for obtaining the
proper clamp voltages:
V
= G ×
= G ×
V
H
V
L
where: V
CH
V
CL
is the high output clamping level
CH
V
is the low output clamping level
CL
G is the gain of the amplifier configuration
V
is the voltage applied to VH (Pin 8)
H
V
is the voltage applied to VL (Pin 5)
L
–2–
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