Capacitor Between DC Lines

[jellybean]

I occasionally see a capacitor placed between two DC voltages (power rails, input to op amps, phantom power for mics, etc.) in circuits as shown below. I'm thinking it for common mode noise, but I'm not sure.

---V+
/
--
-- capacitor
/
----V+ or V-

Could some elaborate (positively)? Thanks.

 

[betwixt]

You are correct. It is for noise reduction and sometimes for keeping RF out of circuits so they don't pick up mobile phones and the likes.
The only exception I can think of is in power supplies where you sometimes see a capacitor between rails to provide 'kick start'. This is where it is necessary to have a temporary voltage one one rail while the power regulator starts up. As the capacitor charges up, the higher voltage it pulls the lower rail up for moment and gives time for it to start the circuit working.

Brian.

 

[trav1s]

Basically, this capacitor acts as a simple passive voltage regulator. If Vdd and Vss remain constant the capacitor is charged and basically invisible to the circuit. A voltage ripple applied to Vdd is synonymous with AC frequencies being present at Vdd. Since a capacitor acts as a low-pass filter, it will help regulate the voltage by charging and discharging when voltage ripples arise.

 

[jellybean]

Thanks for the replies. I instinctively thought it was CM noise, but I honestly don't "see" it - the charging discharging action of the cap and shunting of the noise, particularly if both ends are at the same potential.

It's also not clear from the second post. Perhaps it depends on what's on the other side (input) of the circuit. Here is an example in Figure 6.
**broken link removed**

 

[betwixt]

Assuming you mean C5 and C6, they are an attempt to keep the same DC level and same impedance on both output wires. It isn't a particularly good solution, driving them anti-phase as in figure 7 is better because it allows the receiving end to cancel common mode interference as well.

Brian.

 

[jellybean]

I’m actually referring to C8 1nF. Lines 1 and 2 are both 48V phantom power lines from a preamplifier. C5 and C6 are blocking caps to prevent DC getting into the preamplifier stage.

 

[betwixt]

As I see it:
Line 1 = Ground and negative line
Line 2 = Audio output and positive line
Line 3 = Positive line

I think the reason for using 2 & 3 in the way they are is to keep the wiring impedance matched as closely as possible but it would work better if they both carried signal as it would improve common mode rejection. C8 is effectively across the signal and power so I would assume it is there to reduce RF pick-up. One of the biggest problems with phantom power, especially along long cables is mobile phones, either in calls or just doing their periodic location check. The RF doesn't doesn't get passed along the cable but the effect of it being rectified causes a shift in DC conditions at the amplifier causing the 'chainsaw' noise.

Brian.

 

[jellybean]

betwixt,

Thanks for the feedback. I think I see it now, and I'm a little embarrassed. Probably because I didn't look at the circuit all that closely and noticed that line 2 only has a DC current in it. I had assumed that in order to make this a balanced input, it was tapped off the emitter of the output amplifier Q4 - some configurations for electret to XLR actually do that.

Line 2 feeds back to the DC source, which is ideally low impedance, and should look like a short to any RF signals. Any RF on line 1 will get shunted through the 1nF cap C8 through to line 2 and to the DC source, thus bypassing the input of the preamplifier. So it would help with the chopper (chainsaw) noise that you described. I think that’s how it works…