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HS-1135RH Datasheet(PDF) 2 Page - Intersil Corporation

Part No. HS-1135RH
Description  Radiation Hardened, High Speed, Low Power Current Feedback Amplifier with Programmable Output Limiting
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Maker  INTERSIL [Intersil Corporation]
Homepage  http://www.intersil.com/cda/home

 2 page
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Clamp Operation
The HS-1135RH features user programmable output clamps
to limit output voltage excursions. Clamping action is obtained
by applying voltages to the VH and VL terminals (pins 8 and 5)
of the amplifier. VH sets the upper output limit, while VL sets
the lower clamp level. If the amplifier tries to drive the output
above VH, or below VL, the clamp circuitry limits the output
voltage at VH or VL ( the clamp accuracy), respectively. The
low input bias currents of the clamp pins allow them to be
driven by simple resistive divider circuits, or active elements
such as amplifiers or DACs.
Clamp Circuitry
Figure 1 shows a simplified schematic of the HS-1135RH
input stage, and the high clamp (VH) circuitry. As with all
current feedback amplifiers, there is a unity gain buffer (QX1
- QX2) between the positive and negative inputs. This buffer
forces -IN to track +IN, and sets up a slewing current of (V-
IN -VOUT)/RF. This current is mirrored onto the high
impedance node (Z) by QX3-QX4, where it is converted to a
voltage and fed to the output via another unity gain buffer. If
no clamping is utilized, the high impedance node may swing
within the limits defined by QP4 and QN4. Note that when the
output reaches it’s quiescent value, the current flowing
through -IN is reduced to only that small current (-IBIAS)
required to keep the output at the final voltage.
Tracing the path from VH to Z illustrates the effect of the
clamp voltage on the high impedance node. VH decreases
by 2VBE (QN6 and QP6) to set up the base voltage on QP5.
QP5 begins to conduct whenever the high impedance node
reaches a voltage equal to QP5’s base + 2VBE (QP5 and
QN5). Thus, QP5 clamps node Z whenever Z reaches VH.
R1 provides a pull-up network to ensure functionality with
the clamp inputs floating. A similar description applies to the
symmetrical low clamp circuitry controlled by VL.
When the output is clamped, the negative input continues to
source a slewing current (ICLAMP) in an attempt to force the
output to the quiescent voltage defined by the input. QP5 must
sink this current while clamping, because the -IN current is
always mirrored onto the high impedance node. The clamping
current is calculated as (V-IN - VOUT)/RF. As an example, a
unity gain circuit with VIN =2V, VH = 1V, and RF = 510Ω would
have ICLAMP = (2-1)/510Ω = 1.96mA. Note that ICC will
increase by ICLAMP when the output is clamp limited.
Clamp Accuracy
The clamped output voltage will not be exactly equal to the
voltage applied to VH or VL. Offset errors, mostly due to VBE
mismatches, necessitate a clamp accuracy parameter which is
found in the device specifications. Clamp accuracy is a function
of the clamping conditions. Referring again to Figure 1, it can
be seen that one component of clamp accuracy is the VBE
mismatch between the QX6 transistors, and the QX5 transistors.
If the transistors always ran at the same current level there
would be no VBE mismatch, and no contribution to the
inaccuracy. The QX6 transistors are biased at a constant
current, but as described earlier, the current through QX5 is
equivalent to ICLAMP. VBE increases as ICLAMP increases,
causing the clamped output voltage to increase as well. ICLAMP
is a function of the overdrive level (V-IN -VOUTCLAMPED) and
RF,so clamp accuracy degrades as the overdrive increases, or
as RF decreases. As an example, the specified accuracy of
±60mV for a 2X overdrive with RF = 510Ω degrades to ±220mV
for RF = 240Ω at the same overdrive, or to ±250mV for a 3X
overdrive with RF = 510Ω.
Consideration must also be given to the fact that the clamp
voltages have an effect on amplifier linearity. The
“Nonlinearity Near Clamp Voltage” curve in the data sheet
illustrates the impact of several clamp levels on linearity.
Clamp Range
Unlike some competitor devices, both VH and VL have usable
ranges that cross 0V. While VH must be more positive than VL,
both may be positive or negative, within the range restrictions
indicated in the specifications. For example, the HS-1135RH
could be limited to ECL output levels by setting VH = -0.8V and
VL = -1.8V. VH and VL may be connected to the same voltage
(GND for instance) but the result won’t be in a DC output
voltage from an AC input signal. A 150 - 200mV AC signal will
still be present at the output.
Recovery from Overdrive
The output voltage remains at the clamp level as long as the
overdrive condition remains. When the input voltage drops
below the overdrive level (VCLAMP/AVCL) the amplifier will
return to linear operation. A time delay, known as the
Overdrive Recovery Time, is required for this resumption of
linear operation. The plots of “Unclamped Performance” and
“Clamped Performance” highlight the HS-1135RH’s

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