ispPAC10-01PI Lattice Semiconductor, ispPAC10-01PI Datasheet - Page 9

• Manufacturer Part Number: ispPAC10-01PI
• Description: In-System Programmable Analog Circuit
• Manufacturer: Lattice Semiconductor

Figure 1. FilSum (Filtering/Summation) PACblock
Diagram
Each FilSum PACblock actually employs three instrument
amplifier (IA) PACells: two at the input (IA1 and IA2) and one
as a feedback element around the op amp (IAF). The
instrument amplifier PACells all have differential I/O and
convert an input voltage to an output current (refer to Figure
2). This type of amplifier is sometimes referred to as an
operational transconductance amplifier or OTA. When a
differential input voltage is applied to these IAs, it is
converted to a current proportional to the input signal.
Because an AC signal common to both of the high
impedance inputs of the IA does not create a net differ-
ence in the input signal, it is rejected by the amplifier. This
characterizes the function of what is commonly known as
an instrument amplifier and is a very desirable property
because it acts to preserve the integrity of small signals
in the presence of otherwise overwhelming noise.
Figure 2. Instrument Amplifier PACell
The two input instrument amplifiers have a program-
mable transconductance (g
2 A/V and 20 A/V with programmable input polarity,
whereas the feedback amplifier is fixed at 2 A/V. The IA
PACells exhibit extremely high input impedance so they
don’t load circuitry driving them and their outputs can be
enabled or disabled under E
switching them in and out of the FilSum PACblock cir-
cuitry. These simple characteristics permit a great deal of
Theory of Operation (Continued)
V IN
V IN
V IN+
V IN+
V IN-
V IN-
g m1
g m2
V IN+
V IN-
IA1
IA2
V IN
g m3
IAF
g m
m
) value in 10 steps between
2
CMOS control, effectively
C F
C F
I M
I P
V OUT+
V OUT-
V OUT
9
functionality: Signals can be summed, the resistive am-
plifier feedback can be removed to create an integrator,
the sign of PACblock transfer function can be changed
without changing the input or output loading characteris-
tics. The FilSum PACblock can precisely filter, amplify or
attenuate signals, always maintaining the high imped-
ance input qualities of instrumentation amplifiers.
FilSum PACblock Operation
All ispPAC10 inputs are differential, the input signal being
the difference between input amplifier (IA) PACell pins
V
mode value of the input is ignored, and as long as the
inputs are not within one volt of the supply rails, the part
is in its linear operating region. As the input signal range
exceeds these limits, distortion begins to increase until
clipping occurs. This is discussed further in the advanced
topics section.
The output is also differential, being the difference be-
tween output amplifier (OA) PACell pins V
V
of the supply rails under minimum load. The output has
short circuit protection and is capable of driving resistive
loads as low as 300
1000pF. The output common mode voltage is maintained
at VREF
level. That is, the output amplifier PACell “re-references”
the common mode level of the input signal. This is
accomplished by continuously sensing the output com-
mon mode voltage and comparing it to VREF
shown in Figure 3, and makes it possible to use an
individual FilSum PACblock as a VREF
discussed in the section titled “Using VREF
Figure 3. Output VREF
Input Offset Auto-Calibration. A unique feature of the
ispPAC10 is its ability to automatically calibrate itself to
achieve very low offset error. This is done utilizing on-
chip circuitry to perform an auto-calibration (auto-cal)
IN+
OUT-
(Positive Input) and V
. The output maintains high linearity to within 100mV
IAF
OUT
Specifications ispPAC10
independent of the input common mode
C F
C F
OUT
or capacitances as large as
IN-
Re-Referencing
(Minus Input). The common
VCM IN (2.5V)
OUT
reference as
OUT
OUT+
”.
V OUT
OUT
and
as