User manual

MCP6V01 Input Offset Demo Board User’s Guide
DS51801A-page 12 © 2009 Microchip Technology Inc.
TABLE 2-2: ESTIMATES
Obviously, other values of T
A
, V
DD
, … can be used instead, with the proper adjustments
to these equations.
2.7 REDUCING THE MEASUREMENT NOISE
The noise seen in the measurements is a result of the design choices made for the
MCP6V01 Input Offset Demo Board. The components R12 and C5 set a lowpass pole
at 0.16 Hz, which gives reasonable noise performance (±0.2 µV
PK
referred to the input
of the DUT) and settling time (1 to 2 seconds).
To achieve lower noise in your results, average many measurements together. For
instance, measuring the output (V
M
–V
DD
/2) once a second for 16 seconds (16
samples) should produce an estimate with noise √16 = 4 times lower (i.e., ±0.05 µV
PK
).
There is a practical limit on increasing the sample rate; the noise does not improve
significantly after a certain point. The analog lowpass pole at 0.16 Hz causes closely
spaced samples to be correlated. To avoid the overhead caused by sampling too fast,
keep the sampling period near or above the pole’s time constant (1.0s); this gives a
minimum sample rate of 1 sample per second.
Operating Inputs Estimate
V
DD
(V)
T
A
(°C)
Equations (Note 1) Units
1.8 and 5.5 -40 to +125 1/A
OL
= 0, by assumption µV/V
+25 1/PSRR = (V
OST_2
–V
OST_5
)/(3.7V) µV/V
5.5 -40 V
OS
=V
OST_8
µV
+25 V
OS
=V
OST_2
µV
+85 V
OS
=V
OST_9
µV
+125 V
OS
=V
OST_10
µV
-40 to +125 ΔV
OS
/ΔT
A
=(V
OST_10
–V
OST_8
) / (165°C) µV/°C
+25 1/CMRR = {(V
OST_4
–V
OST_3
)/(4.5V) µV/V
1.8 +25 V
OS
=V
OST_5
µV
1/CMRR = (V
OST_7
–V
OST_6
)/(1.2V) µV/V
Note 1: V
OST_k
is calculated from V
Mk
using Equation 2-1.
Note: Sampling much faster than 1 SPS will not improve the averaged noise of
this board’s output significantly.