REV. B
–16–
AD8036/AD8037
Operation of the AD8036 for negative input voltages and nega-
tive clamp levels on VL is similar, with comparator CL control-
ling S1. Since the comparators see the voltage on the +VIN pin
as their common reference level, then the voltage VH and VL are
defined as “High” or “Low” with respect to +VIN. For example,
if VIN is set to zero volts, VH is open, and VL is +1 V, compara-
tor CL will switch S1 to “C,” so the AD8036 will buffer the
voltage on VL and ignore +VIN.
The performance of the AD8036 and AD8037 closely matches
the ideal just described. The comparator’s threshold extends
from 60 mV inside the clamp window defined by the voltages on
VL and VH to 60 mV beyond the window’s edge. Switch S1 is
implemented with current steering, so that A1’s +input makes a
continuous transition from say, VIN to VH as the input voltage
traverses the comparator’s input threshold from 0.9 V to 1.0 V
for VH = 1.0 V.
The practical effect of these nonidealities is to soften the transition
from amplification to clamping modes, without compromising
the absolute clamp limit set by the CLAMPIN circuit. Figure 7
is a graph of VOUT vs. VIN for the AD8036 and a typical output
clamp amplifier. Both amplifiers are set for G = +1 and VH = 1 V.
The worst case error between VOUT (ideally clamped) and VOUT
(actual) is typically 18 mV times the amplifier closed-loop gain.
This occurs when VIN equals VH (or VL). As VIN goes above
and/or below this limit, VOUT will settle to within 5 mV of the
ideal value.
In contrast, the output clamp amplifier’s transfer curve typically
will show some compression starting at an input of 0.8 V, and
can have an output voltage as far as 200 mV over the clamp limit.
In addition, since the output clamp in effect causes the am-
plifier to operate open loop in clamp mode, the amplifier’s out-
put impedance will increase, potentially causing additional errors.
The AD8036’s and AD8037’s CLAMPIN input clamp architec-
ture works only for noninverting or follower applications and,
since it operates on the input, the clamp voltage levels VH and
VL, and input error limits will be multiplied by the amplifier’s
A
B
C
S1
RF
140
A B C
0 1 0
1 0 0
0 0 1
S1
VIN > VH
VL ≤ VIN ≤ VH
VIN < VL
–VIN
+VIN
VH
VL
VOUT
+1
CH
CL
A1
A2
+1
Figure 6. AD8036/AD8037 Clamp Amp System
0
5
10
15
20
25
R
SERIES
–
CL– pF
40
30
20
10
Figure 5. Recommended RSERIES vs. Capacitive Load
INPUT CLAMPING AMPLIFIER OPERATION
The key to the AD8036 and AD8037’s fast, accurate clamp and
amplifier performance is their unique patent pending CLAMPIN
input clamp architecture. This new design reduces clamp errors
by more than 10
× over previous output clamp based circuits, as
well as substantially increasing the bandwidth, precision and
versatility of the clamp inputs.
Figure 6 is an idealized block diagram of the AD8036 connected
as a unity gain voltage follower. The primary signal path com-
prises A1 (a 1200 V/
s, 240 MHz high voltage gain, differential
to single-ended amplifier) and A2 (a G = +1 high current gain
output buffer). The AD8037 differs from the AD8036 only in
that A1 is optimized for closed-loop gains of two or greater.
The CLAMPIN section is comprised of comparators CH and
CL, which drive switch S1 through a decoder. The unity-gain
buffers in series with +VIN, VH, and VL inputs isolate the input
pins from the comparators and S1 without reducing bandwidth
or precision.
The two comparators have about the same bandwidth as A1
(240 MHz), so they can keep up with signals within the useful
bandwidth of the AD8036. To illustrate the operation of the
CLAMPIN circuit, consider the case where VH is referenced to
1 V, VL is open, and the AD8036 is set for a gain of +1, by con-
necting its output back to its inverting input through the recom-
mended 140
feedback resistor. Note that the main signal path
always operates closed loop, since the CLAMPIN circuit only
affects A1’s noninverting input.
If a 0 V to 2 V voltage ramp is applied to the AD8036’s +VIN
for the connection just described, VOUT should track +VIN
perfectly up to 1 V, then should limit at exactly 1 V as +VIN
continues to 2 V.
In practice, the AD8036 comes close to this ideal behavior. As
the +VIN input voltage ramps from zero to 1 V, the output of the
high limit comparator CH starts in the off state, as does the out-
put of CL. When +VIN just exceeds VIN (ideally, by say 1
V,
practically by about 18 mV), CH changes state, switching S1
from “A” to “B” reference level. Since the + input of A1 is now
connected to VH, further increases in +VIN have no effect on the
AD8036’s output voltage. In short, the AD8036 is now operat-
ing as a unity-gain buffer for the VH input, as any variation in
VH, for VH > 1 V, will be faithfully reproduced at VOUT.