MCP651 Input Offset Evaluation Board User’s Guide
DS51834A-page 14
©
2009 Microchip Technology Inc.
2.4
OPERATING CONDITIONS
The MCP651 Input Offset Evaluation Board works most effectively at room
temperature (near 25°C). Measurements at other temperatures should be done in an
oven where the air velocity is minimal. Table 2-1 shows the various DUT voltages (as
described in the data sheet), their nominal values and ranges, and how to convert to
the voltages needed on the MCP651 Input Offset Evaluation Board.
TABLE 2-1:
CONVERTING VOLTAGES FOR THE BOARD
Once the MCP651 Input Offset Evaluation Board is powered up, the switches can be
set for the desired operation. S1 (a normally off push-button switch) starts a calibration
event (CAL), internal to the DUT, when pushed. S2 (top position) sets the gain of the
amplifier (G
M
) either high or low. See Table 2-2 for details.
TABLE 2-2:
SWITCH OPERATION
The gain is usually set low. It can be set high just after a calibration event, before
changing the DUT’s bias point, to obtain more accurate results for the calibrated offset
voltage.
The POT (R
CAL
or R
43
) adjusts V
CALX
. This voltage is where the DUT’s common mode
input voltage set during a calibration event (initiated by pushing S1). Adjusting this POT
does not have an effect on the circuit’s behavior until the CAL switch (S1) is pushed.
Single Supply Voltages (V)
Conversion
Equations (V)
Data Sheet
Symbol
Nominal
Range
V
DD
2.5 or 5.5
2.5 to 5.5
V
DDX
←
V
DD
– V
CM
V
SS
0
0
V
SSX
←
V
SS
– V
CM
V
CM
V
DD
/3
V
SS
– 0.3 to V
DD
– 1.3
0
←
V
CM
– V
CM
V
OUT
V
DD
/2
V
SS
+ 0.2 to V
DD
– 0.2
V
OUTX
←
V
OUT
– V
CM
V
L
V
DD
/2
V
SS
to V
DD
V
LX
←
V
L
– V
CM
V
CAL
V
DD
/3
V
SS
+ 0.1 to V
DD
– 1.4
V
CALX
←
V
CAL
– V
CM
CAL/CS
V
SS
V
SS
to V
DD
Note 1:
At T
A
= +25°C. See the data sheet for changes in V
CM
range vs. T
A
.
2:
Set the desired V
OUT
voltage at the V
COX
input; the integrator then forces V
OUT
to
be the same voltage.
3:
When the V
CAL
pin left open. However, this board always has the POT (R43)
connected, so V
CALX
is never truly open.
4:
The circuit forces CAL/CS to stay within its range (as long as the supply voltages
are constant when the CAL switch is activated). Normally, the part is on.
5:
These numbers are for the MCP651 op amp.
Switch
Input
Result
S1
No Input
Normal Operation
Pushed
Calibration event started in DUT
S2
Top Switch to the left
High Gain (1998 V/V)
Top Switch to the right
Low Gain (201 V/V)
(Bottom Switch)
(Don’t Care)
