SHC615 Datasheet, PDF (13/19 Pages) Burr-Brown (TI) – Wide-Bandwidth, DC RESTORATION CIRCUIT

DISCUSSION OF
PERFORMANCE
The SHC615, which contains a wideband Operational
Transconductance Amplifier and a fast sampling compara-
tor, represents a complete subsystem for very fast and
precise DC restoration, offset clamping and correction to
GND or to an adjustable reference voltage, and low fre-
quency hum suppression of wideband operational or buffer
amplifiers.
Although the IC was designed to improve or stabilize the
performance of complex, wideband video signals, it can also
be used as a sample and hold amplifier, high-speed integra-
tor, peak detector for nanosecond pulses, or demodulator or
modulator for pulse code transmission systems. A wideband
Operational Transconductance Amplifier (OTA) with a high-
impedance cascode current source output and a fast and
precise sampling comparator set a new standard for high-
speed sampling applications.
Both can be used as stand-alone circuits or combined to
create more complex signal processing stages like sample
and hold amplifiers. The SHC615 simplifies the design of
input amplifiers with high hum suppression, clamping or
DC-restoration stages in professional broadcast equipment,
high-resolution CAD monitors and information terminals,
signal processing stages for the energy and peak value of
small and fast nanoseconds pulses, and eases the design of
high-speed data acquisition systems behind a CCD sensor or
in front of an analog-to-digital converter.
An external resistor, R_Q, allows the user to set the quiescent
current. R_Q is connected from Pin 1 (I_Q adjust) to –V_CC. It
determines the operating currents of both the OTA and
comparator sections and controls the bandwidth and AC
behavior as well as the transconductance of both sections.
Besides the quiescent current setting feature, the Propor-
tional-to-Absolute-Temperature (PTAT) supply increases the
quiescent current vs temperature and keeps it constant over
a wide range of input voltages. This variation holds the
transconductance g_m of the OTA and comparator relatively
constant vs temperature. The circuit parameters listed in the
specification table are measured with R_Q set to 300Ω, giving
a nominal quiescent current at ±15mA. The circuit can be
totally switched-off with a current flowing into Pin 1.
output (emitter), and the high-impedance current output
(collector).
The OTA consists of a complementary buffer amplifier and
a subsequent complementary current mirror. The buffer
amplifier features a Darlington output stage and the current
mirror has a cascoded output. The addition of this cascode
circuitry increases the current source output resistance to
1MΩ and the open-loop gain to typically 96dB. Both fea-
tures improve the OTAs linearity and drive capabilities. Any
bipolar input voltage at the high impedance base has the
same polarity and signal level at the low impedance buffer
or emitter output. For the open-loop diagrams the emitter is
connected to GND and then the collector current is deter-
mined by the product voltage between base and emitter
times the transconductance. In application circuits (Figure
2b.), a resistor R_E between emitter and GND is used to set
the OTA transfer characteristics. The following formulas
describe the most important relationships. r_E is the output
impedance of the buffer amplifier (emitter) or the reciprocal
of the OTA transconductance. Above ±5mA, collector cur-
rent, I_C, will be slightly less than indicated by the formula.
I_C
=
V _IN
r_E + R_E
R_E
=
V _IN
I_C
– r_E
The R_E resistor may be bypassed by a relatively large
capacitor to maintain high AC gain. The parallel combina-
tion of R_E and this large capacitor form a high pass filter
enhancing the high frequency gain. Other cases may require
a RC compensation network parallel to R_E to optimize the
high-frequency response. The full power bandwidth mea-
sured at the emitter achieves 620MHz. The frequency re-
sponse of the collector is directly related to the resistor’s
value between collector and GND; it decreases with increas-
ing resistor values, because it forms a low-pass network with
the OTA C-output capacitance.
Figure 1 shows a simplified block and circuit diagram of
the SHC615 OTA. Both the emitter and the collector
outputs offer a drive capability of ±20mA for driving low
impedance lines or inputs. Connecting the collector to the
emitter in a direct-feedback buffer configuration increases
the drive capability to ±40mA. The emitter output is not
current-limited or protected. Momentary shorts to GND
should be avoided, but are unlikely to cause permanent
damage.
OPERATIONAL
While the OTA’s function and labeling looks similar to
that of transistors, it offers essential distinctive differences
TRANSCONDUCTANCE
AMPLIFIER (OTA)
and improvements: 1) The collector current flows out of
the C terminal for a positive B-to-E input voltage and into
it for negative voltages; 2) A common emitter amplifier
SECTION AND OVERVIEW
The symbol for the OTA section is similar to that of a
bipolar transistor, and the self-based OTA can be viewed as
a quasi-ideal transistor or as a voltage-controlled current
source. Application circuits for the OTA look and operate
much like transistor circuits—the bipolar transistor, also, is
a voltage-controlled current source. Like a transistor, it has
three terminals: a high-impedance input (base) optimized for
a low input bias current of 0.3µA, a low-impedance input/
operates in non-inverting mode while the common base
operates in inverting mode; 3) The OTA is far more linear
than a bipolar transistor; 4) The transconductance can be
adjusted with an external resistor; 5) Due to the PTAT
biasing characteristic the quiescent current increases as
shown in the typical performance curve vs temperature
and keeps the AC performance constant; 6) The OTA is
self-biased and bipolar; and, 7) The output current is zero
for zero differential input voltages. AC inputs centered at
zero produce an output current centered at zero.
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SHC615