Microchip Technology MCP6N16, Руководство пользователя, Страница: 21 / 36

Installation and Operation

2015 Microchip Technology Inc. DS50002365A-page 21
2.4.3.1 INPUT AC-COUPLING
With resistors R6 and R9 (both 0 Ω ) populated, the signal inputs on the evaluation board
are DC coupled to the inputs of the instrumentation amplifier MCP6N16. Alternatively,
the inputs can be configured for AC coupling. For this, replace resistors R6 and R9 with
ceramic capacitors (0.1 µF, or as required). In this AC-coupling configuration, it is
important to provide a DC bias path for the inputs of the instrumentation amplifier. This
is accomplished with resistors R13 and R14, which are already installed. They can be
either referenced to ground or an external common-mode voltage (V
CM
) by setting the
jumper J10 accordingly. The resistor along with the coupling capacitors will also result
in a high-pass filter; for example using 0.1 µF capacitors and 200 k
Ω resistor will set
the -3 dB frequency at about 8 Hz.
2.4.3.2 INPUT AND OUTPUT FILTERING
The MCP6N16 features internal EMI filters on all four of its inputs that are very effective
suppressing high-frequency signals from interfering and causing unwanted offset
voltages. Those internal filters may already be sufficient for some applications and the
sensor can be connected directly to the inputs of the MCP6N16. The evaluation board
includes additional external RC filtering comprised of common-mode and
differential-mode filters which will limit the input signal bandwidth according to
Equation 2-1 .
EQUATION 2-1:
Note that capacitors C6 and C9 have been selected with a lower tolerance of 5%,
instead of the typical 10%, to improve time constant matching between R7C6 and
R10C9 and consequently limit the CMRR degradation caused by such mismatches.
The -3 dB frequency for these filters is about 8 kHz. The MCP6N16-100 maintains a
very high common-mode rejection, CMRR of > 100 dB out to 100 kHz. This allows for
a relatively high corner frequency to be chosen for this filter and therefore reduces the
series resistor (i.e. R7 and R10) value, which may otherwise cause unwanted offset
and noise contributions at the input of the high-gain instrumentation amplifier.
It is recommended to keep the value of C7 at least ten times larger than C6 and C9 to
reduce the effects of the time constant mismatch and improve performance.
EQUATION 2-2:
The differential-mode input filter has its -3 dB frequency corner at about 378 Hz. Note
that the filter will start affecting the gain at a much lower frequency, for example the
input signal will be lowered by 1% at about 54 Hz.
Note:
When using resistors R13 and R14, the input impedance at the AIN+ and
AIN- is determined by the value of these resistors. Removing the resistors
will restore the high input impedance provided by the MCP6N16, but will
also eliminate the option of using an external common-mode voltage (V
CM
)
through J2.
fCM
1
2

R7 C6

 
----------------------------------
1
2

R10 C9

 
------------------------------------- = =
Common-Mode filter:
fDIFF
1
2

R7 R10 +
 
C7
C6
2
------- +
 
 
 
 
----------------------------------------------------------------------- =
fDIFF
1
2

R 2C7 C6 +
   
----------------------------------------------- =
Differential-Mode filter:
with R7 = R10: