Hi Mike, I wanna ask the question that in normal applications when inputs are not needed for test reasons, is it OK to tie the un-used inputs of TTL or NAND devices directly to Ground or one must always need to use the pull-down resistors. Regards, ADEEL MALIK -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Mike Brown Sent: Sunday, January 05, 2003 1:08 AM To: deepakkatkoriain@xxxxxxxxxxx Cc: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Regarding Pull down Deepak, CMOS and TTL are different things when it comes to pulldown resistors. First, I assume that the resistors on the inputs are needed for test reasons, otherwise the inputs could be tied directly to the ground plane. I'm also going to assume that the trace between the input pin and the pulldown resistor is relatively short, so that noise pickup (crosstalk) onto that line is minimal. The job of the pulldown R is to sink any current (leakage or otherwise) from the input pin to the ground plane, with a minimum DC drop, to establish the DC level at that input. Current must be pulled from a true TTL input to bring it to a low voltage. This paragraph does not apply to "TTL-compatible" CMOS inputs. Depending on the particular TTL family, as much as 2 ma might be required. In any case, this current flows through the pulldown R and the drop across R limits the low level input voltage at the gate. Smaller is better in this case, within the limits of the tester's ability to drive the R to a "1" level. CMOS does not have the large sink current requirement (a few microamps at most) so a larger value R may be used as a pulldown, consistent with a low DC drop across the R. The amplitude of the ground plane noise, either absolute relative to some arbitrary point, is not very important with respect to this discussion. The plane noise at the pulldown resistor ground terminal, relative to the ground terminal of the gate with the pulled-down input, appears on that gate input as signal. So long as the noise is less than the switching threshold of the gate minus the DC drop of the pulldown, you are in business. Any crosstalk is superimposed on the gate's DC input level. This is more of an issue with true TTL than with CMOS inputs, due to the relatively low Vil spec of TTL and the unavoidable DC drop on its pulldown R. Thus the "short wire" assumption above. A numerical example: True TTL with an Iil of 2 mA, with 100 ohm pulldown. The max drop across the 100 ohms is 200 mV. The threshold is, say, 800 mV. This leaves a minimum noise margin of 600 mV, for coincident ground and crosstalk noise. Regards Mike Deepak Vinod wrote: >Hi All, >I have following simple doudt regarding pull down of >unused input of CMOS/TTL. >If my ground plane have 200-300mv (or ideal 0 >volt)noise, then in such case what should be the value >of pull down resistor for both different family. > >I got one feedback, but i am not sure about it. >He mention that .... >1) The value should be low (around 100 ohms), so it >can not pick up the noise and give false trigger to >input. > >what is my doudt is, if R will be low then it will >consumed more ripple current from plane and generate >faulty voltage. > >What will be the preferable value of R (100 ohms,1K >ohms, 4.7Kohms)? >Please reply soon. > >Thanks in Anticipation. >Deepak ><snip> > > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu