Datasheet

4.4 Using the DRV602 as a Differential Input, Second-Order Low-Pass Filter
R
2
R
2
R
1
R
3
C
1
C
1
C
3
R
1
R
3
C
3
-In
+In
C
2
DRV602
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The resistor values should be low value to achieve low noise, but should be of high enough value to
obtain a small size ac-coupling capacitor. With the proposed values of 33 k Ω , 68 k Ω , and 100 k Ω , a
dynamic range (DYR) of 102 dB can be achieved with a small 1- µ F input ac-coupling capacitor.
The MFB filter structure demands an op amp that is unity-gain stable at high frequencies; this requirement
can be relaxed by adding the C
X
capacitor value to be equal to C1. The DRV602 is unity-gain stable, but
stray capacitance and inductance from the PCB layout can affect the phase margin. Therefore, we
recommend adding C
X
.
The single-ended input, second-order filter described in Section 4.3 can easily be extended to have a
differential input, as shown in Figure 4 .
Figure 4. Differential Input, Second-Order Active Low-Pass Filter
C
2
can be split into separate capacitors to ground with the double value; this technique increases the
common-mode filtering. Another capacitor, C
X
, set equal to C
1
can continue to be used from the negative
input to ground to limit the high-frequency gain to 2.
As with the single-ended input, the differential input filter component values can be calculated with the
help of the TI FilterPro active filter design program available on the TI web site.
DRV602EVM2 6 SLOU248A – December 2008 – Revised January 2009
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