adcclock
I am using following circuit and crystal is connected to XTAL2 and XTAL1 pin as shown below.
Following IC is LPC2138, shown in image.
I am using ADC chip ( DDC112 ) for A to D conversion. For ADC IC, CLK pin is needed. If I want to use same crystal (Y1 in the image) for ADC, I need to assign this pin to CLK pin of ADC.
Which pin should I connect to CLK pin of ADC, XTAL2 or XTAL1 (if crystal frequncy is same for both chip) ?
I'd like to chime in with a bit of perspective: you're not really dealing with anything high speed here. You can really do whatever you want at these frequencies and get perfectly fine signal integrity. A couple of MHz, even a couple tens of MHz, won't cause any real trouble.
Especially on the crystal side of things, you don't have to worry about a thing. I've had layouts where the crystal was placed several inches away from an MCU, and it worked just fine. I probed it, it looks just like the crystal is tightly coupled. High speed design is 300MHz and up - or high impedance from about 100MHz. That is roughly the point where routing can start causing clock skew, reflections and where your PCB traces won't behave as resistive elements anymore.
In any noise-sensitive design, you want to consider that:
Well you haven't said what your supply voltage is or what is wrong with the output but this might be a clue: -
For the inductor, the higher the Q the less damping there will be in the L-C filter. This can be a good thing (and a bad thing i.e. it may create unwanted overshoot with transients).
The self resonant frequency is specified because the inductor has parasitic parallel capacitance and this will form a resonant circuit with the inductance - this area generally is to be avoided but in your circuit (low pass filter) I don't think a SRF above a GHz will be much trouble. Whatever the SRF is, it will form a parallel blocking impedance at SRF.
Best Answer
You can use XTAL2 as your ADC's clock source, but you may need a buffer between XTAL2 and your ADC's CLK pin, such as a Schmitt-trigger 74HC1G14.
Update:
As the OP's question in comment:
Per LPC2183's datasheet, XTAL2's output voltage ranges from -0.5V to 1.95V. This voltage level may not high enough to drive your ADC's CLK input. And this may also slow down the edge rate of the clock signal.