Datasheet

MCP6V31/1U
DS25127A-page 20 © 2012 Microchip Technology Inc.
4.1.3 INTERMODULATION DISTORTION
(IMD)
These op amps will show intermodulation distortion
(IMD) products when an AC signal is present.
The signal and clock can be decomposed into sine
wave tones (Fourier series components). These tones
interact with the zero-drift circuitry’s non-linear
response to produce IMD tones at sum and difference
frequencies. Each of the square wave clock’s
harmonics has a series of IMD tones centered on it.
See Figure 2-36 and Figure 2-37.
4.2 Other Functional Blocks
4.2.1 RAIL-TO-RAIL INPUTS
The input stage of the MCP6V31/1U op amps uses two
differential CMOS input stages in parallel. One
operates at low common mode input voltage (V
CM
,
which is approximately equal to V
IN
+ and V
IN
– in
normal operation) and the other at high V
CM
. With this
topology, the input operates with V
CM
up to V
DD
+0.2V,
and down to V
SS
– 0.15V, at +25°C (see Figure 2-18).
The input offset voltage (V
OS
) is measured at
V
CM
=V
SS
– 0.15V and V
DD
+ 0.2V to ensure proper
operation.
The transition between the input stages occurs when
V
CM
≈ V
DD
–0.9V (see Figure 2-7 and Figure 2-8). For
the best distortion and gain linearity, with non-inverting
gains, avoid this region of operation.
4.2.1.1 Phase Reversal
The input devices are designed to not exhibit phase
inversion when the input pins exceed the supply
voltages. Figure 2-42 shows an input voltage
exceeding both supplies with no phase inversion.
4.2.1.2 Input Voltage Limits
In order to prevent damage and/or improper operation
of these amplifiers, the circuit must limit the voltages at
the input pins (see Section 1.1, Absolute Maximum
Ratings †). This requirement is independent of the cur-
rent limits discussed later on.
The ESD protection on the inputs can be depicted as
shown in Figure 4-4. This structure was chosen to
protect the input transistors against many (but not all)
overvoltage conditions, and to minimize input bias
current (I
B
).
FIGURE 4-4: Simplified Analog Input ESD
Structures.
The input ESD diodes clamp the inputs when they try
to go more than one diode drop below V
SS
. They also
clamp any voltages that well above V
DD
; their
breakdown voltage is high enough to allow normal
operation, but not low enough to protect against slow
overvoltage (beyond V
DD
) events. Very fast ESD
events (that meet the spec) are limited so that damage
does not occur.
In some applications, it may be necessary to prevent
excessive voltages from reaching the op amp inputs;
Figure 4-5 shows one approach to protecting these
inputs. D
1
and D
2
may be small signal silicon diodes,
Schottky diodes for lower clamping voltages or diode
connected FETs for low leakage.
FIGURE 4-5: Protecting the Analog Inputs
Against High Voltages.
Bond
Pad
Bond
Pad
Bond
Pad
V
DD
V
IN
+
V
SS
Input
Stage
Bond
Pad
V
IN
–
V
1
V
DD
D
1
V
OUT
V
2
D
2
U
1
MCP6V3X