why the series resistor?

Discussion in 'The Projects Forum' started by pinnacle06, Sep 28, 2013.

Feb 9, 2007
16
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Hi folks,

I am playing with a circuit at the moment. I have a clock signal going through a digital buffer and fanning out. I am looking at an application circuit. I understand the need for a small series resistor at the output of the buffer. But there is also a 1k series resistor on the digital input of the chip that uses this clock signal. What is the purpose of that 1k resistor? Any ideas?

2. MrChips Moderator

Oct 2, 2009
19,051
6,121
Can you post a schematic?

3. Art AAC Fanatic!

Sep 10, 2007
805
62
To decouple the buffer's capacitance from the crystal's load cap.

Feb 9, 2007
16
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Once I get on my other computer I can post the schematic, I know it seems a little unclear.

These resistors on the output side of the resistor, not the input. It goes like this:

clock buffer output --> small resistor --> PCB trace etc --> 1 k resistor --> digital input of chip. All in series, nothing else on the line actually.

Thanks again!

5. Art AAC Fanatic!

Sep 10, 2007
805
62
Sounds like the same thing:

The inputs would normally clocks?
They are there, I think, because everything is adding capacitance either end of the buffer chip,
including PCB tracks and the buffer chip itself.
If you keep adding more clocks, in the end, the original clock fails to oscillate.

Last edited: Sep 29, 2013

Apr 30, 2011
1,568
442
It may be a feedback line to set the gain of the buffer, if I'm understanding your description. Post the schematic.

Feb 9, 2007
16
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Here is the basic schematic. I don't think the 1k resistor is there for feedback purposes. Any thoughts on what that 1k is there for?

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8. MrChips Moderator

Oct 2, 2009
19,051
6,121
The 1k resistor serves no purpose except to slow the rise and fall times of the signal.

Feb 9, 2007
16
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hmmm... interesting. Is this a common design approach for this type circuit?

10. MrChips Moderator

Oct 2, 2009
19,051
6,121
No.

What is the device you have labelled as "chip"?

Feb 9, 2007
16
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In this particular case it is a clock input for a digital microphone. But I have seen this 1k resistor on digital input pins for other devices as well such as a clock line for SPI as well. I have been confused as to why it's there. I'll try and dig around a little more and figure it out.

Thanks a lot for the help!

Apr 30, 2011
1,568
442
I suspect we're missing some information such as a connection to Vcc or ground. Considering that the basic function of a resistor in a circuit is usually to divide voltage or limit current, could the present case be a pullup/pulldown or some kind of crude level shifter?

Last edited: Sep 30, 2013

Feb 9, 2007
16
0
That is really all there is to it... there is no other connection on that line but I think it might be a mistake. Since I do not understand it, I am going to build the board with a 0 ohm and experiment with it. Thanks a lot for the help!

14. MrChips Moderator

Oct 2, 2009
19,051
6,121
A series resistor of 33Ω or 50Ω for impedance matching is common for fast signals.

1kΩ resistor in series with the input can be used to protect the inputs if the signal level is higher than the chip's supply voltage. You don't indicate this is the case.

15. WBahn Moderator

Mar 31, 2012
24,480
7,660
If you have fast transitioning signals, it can inject noise into other parts of the circuit (via several mechanisms) or radiate and cause EMI problems. A series resistance, in combination with the parasitic capacitance, can slow the signal edges and reduce the problem. Not necessarily the best way to go about it, but quick, dirty, and cheap.

16. atferrari AAC Fanatic!

Jan 6, 2004
3,314
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In (most or all?) Microchip's PICs, a resistor is reccomended at one of the oscillator inputs when there is an AT strip cut crystal used.

17. Tesla23 AAC Fanatic!

May 10, 2009
373
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The 50R at the output of the HC buffer looks like a source termination resistor as suggested by Mr Chips. This would make sense if the "Digital In" pin of the "chip" is edge sensitive as the source termination, if done correctly, will eliminate ringing.

Google 'source termination clock edge' or similar.