Datasheet

© 2008 Microchip Technology Inc. DS21613C-page 15
MCP616/7/8/9
4.0 APPLICATIONS INFORMATION
The MCP616/7/8/9 family of op amps is manufactured
using Microchip’s state-of-the-art CMOS process,
which includes PNP transistors. These op amps are
unity-gain stable and suitable for a wide range of
general purpose applications.
4.1 Rail-to-Rail Inputs
4.1.1 PHASE REVERSAL
The MCP616/7/8/9 op amp is designed to prevent
phase reversal when the input pins exceed the supply
voltages. Figure 2-36 shows the input voltage exceed-
ing the supply voltage without any phase reversal.
4.1.2 INPUT VOLTAGE AND CURRENT
LIMITS
The ESD protection on the inputs can be depicted as
shown in Figure 4-1. This structure was chosen to
protect the input transistors, and to minimize input bias
current (I
B
). 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 go too far
above V
DD
; their breakdown voltage is high enough to
allow normal operation, and low enough to bypass
quick ESD events within the specified limits.
FIGURE 4-1: Simplified Analog Input ESD
Structures.
In order to prevent damage and/or improper operation
of these op amps, the circuit they are in must limit the
currents and voltages at the V
IN
+ and V
IN
– pins (see
“Absolute Maximum Ratings †” at the beginning of
Section 1.0 “Electrical Characteristics”). Figure 4-2
shows the recommended approach to protecting these
inputs. The internal ESD diodes prevent the input pins
(V
IN
+ and V
IN
–) from going too far below ground, and
the resistors R
1
and R
2
limit the possible current drawn
out of the input pins. Diodes D
1
and D
2
prevent the
input pins (V
IN
+ and V
IN
–) from going too far above
V
DD
, and dump any currents onto V
DD
. When
implemented as shown, resistors R
1
and R
2
also limit
the current through D
1
and D
2
.
FIGURE 4-2: Protecting the Analog
Inputs.
It is also possible to connect the diodes to the left of
resistors R
1
and R
2
. In this case, current through the
diodes D
1
and D
2
needs to be limited by some other
mechanism. The resistors then serve as in-rush current
limiters; the DC current into the input pins (V
IN
+ and
V
IN
–) should be very small.
A significant amount of current can flow out of the
inputs when the common mode voltage (V
CM
) is below
ground (V
SS
). (See Figure 2-36.) Applications that are
high impedance may need to limit the usable voltage
range.
4.1.3 NORMAL OPERATION
The inputs of the MCP616/7/8/9 op amps connect to a
differential PNP input stage. The common mode input
voltage range (V
CMR
) includes ground in single-supply
systems (V
SS
), but does not include V
DD
. This means
that the amplifier input behaves linearly as long as the
common mode input voltage (V
CM
) is kept within the
specified V
CMR
limits (V
SS
to V
DD
–0.9V at +25°C).
4.2 DC Offsets
The MCP616/7/8/9 family of op amps have a PNP input
differential pair that gives good DC performance. They
have very low input offset voltage (±150 µV, maximum)
at T
A
= +25°C, with a typical bias current of -15 nA
(sourced out of the inputs).
There must be a DC path to ground (or power supply)
from both inputs, or the op amp will not bias properly.
The DC resistances seen by the op amp inputs (R
1
||R
2
and R
4
||R
5
in Figure 4-3) need to be equal and less
than 100 kΩ, to minimize the total DC offset.
Bond
Pad
Bond
Pad
Bond
Pad
V
DD
V
IN
+
V
SS
Input
Stage
Bond
Pad
V
IN
–
V
1
MCP61X
R
1
V
DD
D
1
R
1
>
V
SS
– (minimum expected V
1
)
2mA
R
2
>
V
SS
– (minimum expected V
2
)
2mA
V
2
R
2
D
2
R
3