Installation and Operation
©
2009 Microchip Technology Inc.
DS51834A-page 15
2.5
CONVERTING TO OTHER PARAMETERS
2.5.1
Calculating DUT Parameters
The DUT’s total input offset voltage (V
OST
) can be calculated from a measurement as
EQUATION 2-1:
Changing the DUT’s bias voltages or ambient temperature changes V
OST
. Microchip’s
application note AN1177 discusses in detail how these changes in V
OST
are related to
specifications found in our data sheets. The following list summarizes the results:
• Specified Input Offset Voltage:
- V
OS
= Input offset at the specified bias point
• DC Common Mode Rejection Ratio:
- CMRR =
Δ
V
CM
/
Δ
V
OS
• DC Power Supply Rejection Ratio:
- PSRR = (
Δ
V
DD
–
Δ
V
SS
)/
Δ
V
OS
• DC Open-loop Gain:
- A
OL
=
Δ
V
OUT
/
Δ
V
OS
• Input Offset Drift over Temperature:
-
Δ
V
OS
/
Δ
T
A
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
V
OST
V
M
G
A
G
M
(
)
⁄
=
Note:
The data sheet Input Offset Voltage (V
OS
) specification applies to one bias
point and temperature only. The total input offset voltage (V
OST
) includes
V
OS
and changes in input offset as bias voltages and temperature change.
Given:
Then:
V
OST
= 0.5 mV,
V
CM
= 0V
V
OST
= 1.0 mV,
V
CM
= 5V
Δ
V
OST
= 0.5 mV
Δ
V
CM
= 5.0V
CMRR = 5.0V / 0.5 mV
= 10 V/mV
= 80 dB
