Page 58 - Troubleshooting Analog Circuits

ESR, Friend or Foe? 45

rep rates are close to the resonant frequency of the LC network! And remember that
the Z5U capacitors have a poor TC. so that as the circuit warms up, it really is likely
that there will be a temperature where the ringing frequency moves up to be a mul-
tiple of the clock frequency.
The standard solution is to add 2 pF of tantalum electrolytic bypass capacitors or
20 pF of aluminum electrolytic capacitors for every three to five ICs (unless you can
prove that they are unnecessary). That’s a good rule of thumb. The ESR of the elec-
trolytic capacitors, typically 1 R, is essential to damp out the ringing. Some people
say that this ESR is too high to do any good in a bypass capacitor-but they do not
understand the problem. I have read a few ads in which some capacitor manufac-
turers claim that their ceramic bypass capacitors are so good-have such low series
resistance-that ringing is no longer a problem. I find the claims hard to believe. I
invite your comments.

ESR, Friend or Foe?
Specifically, some capacitor manufacturers claim that the series resistance, R,, is so
low that you won’t have a problem with ringing. But low Rs would seem to exacer-
bate the ringing problem. Conversely, I’ve heard that one capacitor manufacturer ia
proposing to market ceramic capacitors whose series Rs has a lower limit-a few
ohms-to help damp out any ringing. I’m going to have to look into that. But if you
have bypass capacitors with a very low Rs. you can lower the Q of the resonator you
have inadvertently constructed around them by adding a resistor of 2.7 to 4.7 R in
series with some of the capacitors. Adding resistance in series with bypass capacitors
might seem a bit silly, but it’s a very useful trick.
High-K ceramic capacitors also can exhibit piezoelectric effects: When you put a
good amount of AC voltage across them, they can hum audibly; and if you rattle or
vibrate them, they can kick out charge or voltage. (Other types can do the same thing,
but high-K types are worse.) Be careful when using these capacitors in a high-vibra-
tion environment.
The capacitance of stable-K capacitors, such as X7R, typically decreases by less
than 15% from the room-temperature value over the -55 to +125 “C range. These
capacitors are general-purpose devices and are usually available in the 100 to
10,000 pF range; in the larger packages, you can get as much as 300,000 pF.
However, you can buy a 10,OOO pF capacitor in either a high-K or a stable-K type;
and you can’t be sure of the kind you’re getting unless you check the catalog and the
part number. Or, measure the capacitance as you heat or cool it.
The last type of ceramic capacitor was originally called “NPO” for Negative-
Positive-Zero, and is now usually called “COG.” Everybody calls them “COG,” (C-
oh-G) but it really is C-zero-G. I’ve seen the EIA document (Ref. 2). The COG / NPO
capacitors have a really high-grade low-K dielectric with a guaranteed TC of less
than +30 ppm/”C. Their dissipation factor, dielectric absorption, and long-term
stability are not quite as good as those of Teflon capacitors but are comparable to
those of other good precision-film capacitors. And the TC is better than almost any-
thing you can buy. So, if you want to make a sample-and-hold circuit usable over the
military temperature range, you’ll find that COG capacitors are more compact and
less expensive then Teflon parts. Many, but not all, ceramic capacitors smaller than
100 pF are made with the COG characteristic. You can get a 22,000 pF COG capacitor
in a 0.3-in.-square package, if you’re willing to pay a steep price.
About every year or so, a customer calls me about a drift problem: His V/F con-
verter has a poor TC, even though he said that he had put in a COG 0.01 pF capacitor