NEDSP900 Development Board
26
RF Filtering
To improve RF immunity a simple PI-Filter (created by C21, C23, L4 Thru L6, C22 and C24) has been fitted
to the front of the circuit, this ensures that all 3 lines have improved RF immunity (even the Ground).
DC Isolation
There are two ways of achieving this, on is to buy expensive Non-Polarised Electrolytic capacitors and
the other to use 2 in series back to back (C37, C40, C35 and C47). The latter approach has been taken for
cost purposes. In the centre of these serial capacitors is a drain down circuit which not only keep the
pre-amplifier stable, but makes sure that the capacitors have a reference point to work against.
Operational Amplifier Gain stage
This uses a standard operational amplifier in its Balanced configuration such that Common Mode
Signals. (I.e. Similar signals that are traveling along both of the inputs) are minimised when amplified.
R25 and R26 form the input resistance to the Op-Amp, R27 and R28 are the feedback resistors.
Gain Setting
The circuit diagram shows the R25 and R28 set to 22k Ohms to give around 26dB gain. Calculating for a
different gain on say the second balanced input:
Gain = R41/R42 = R38/R43 Note: R42 and R43 need to be the same value and likewise R38 and R41 also
need to be the same
Frequency Roll off
If the Gain value is changed, so too will the Frequency roll off given by Capacitors C25 through C28.
These just add a 3dB roll off and have initially been calculated to give a corner frequency of around
10 kHz.
Note: The NEDSP900 DSP has a roll off of around 5 kHz and the circuits that follow have also been
tailored to roll off at no more than around 6 kHz.
Output
The output of the Op-Amp IC3a is protected (and stabilised) using R29, with C49 providing DC Isolation
for the next stage.
