User manual

Installation and Operation
© 2009 Microchip Technology Inc. DS51801A-page 11
Example 2-1 gives an example of how V
OST
changes with the common mode input
voltage (V
CM
).
EXAMPLE 2-1: COMMON MODE CHANGE EXAMPLE
2.6.2 Application
When the common mode voltage (V
CM
) is changed on this demo board, the output
voltage (V
OUT
) is forced to change by the same amount. There is no means provided
to independently change V
CM
and V
OUT
. Thus, it is not possible to separate the
Open-Loop Gain (A
OL
) effect from the CMRR effect using this board.
Since A
OL
is usually much better than CMRR for the MCP6V0X op amps, we can
attribute most of the change to CMRR and ignore A
OL
in most cases. Table 2-1 shows
one possible measurement matrix that will allow the user to estimate key parameters
for the DUT.
TABLE 2-1: MEASUREMENT MATRIX
Based on these measurements, we can make the following estimates, where the
V
OST_k
values are calculated from the measured V
Mk
values (see Equation 2-1):
Given:
Then:
V
OST
=0.5µV, V
CM
=0V
V
OST
=1.0µV, V
CM
=5V
ΔV
OST
=0.5µV
ΔV
CM
=5.0V
CMRR = 5.0V / 0.5 µV
=10V/µV
= 140 dB
Note: V
OUT
cannot be changed independently of V
CM
, so A
OL
and CMRR cannot
be distinguished using this circuit.
Operating Inputs
(Note 1)
Measurement
T
A
(°C)
V
DD
(V)
V
CM
(V)
Symbol Comments
+25 5.5 OFF V
M1
Quick Check
V
DD
/3 V
M2
V
OS
and PSRR
0.5 V
M3
CMRR and A
OL
5.0 V
M4
1.8 V
DD
/3 V
M5
V
OS
and PSRR
0.3 V
M6
CMRR and A
OL
1.5 V
M7
-40 5.5 V
DD
/3 V
M8
V
OS
at temperature and ΔV
OS
/ΔT
A
+85 V
DD
/3 V
M9
+125 V
DD
/3 V
M10
Note 1: V
SS
=GND=0V. V
CM
= OFF means that the its power supply is off (V
CM
=V
DD
/2).
V
OUT
≈ V
CM
.