From: Winfield Hill
Subject: Re: how to reduce noise from 555 timer...???
Date: 20 Dec 2002 01:21:04 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
> I am designing a weighting machine using Strain gauge, the design look like
> Strain Gauge -> wheatstone bridge -> Instrumentation Amplifier (MAX4460,
> gain=10,000) -> Analog-2-Digital convertor (ADC0801, Vref=2.5V) -> Shift
> register (74LS574, CLK=3Hz from 555 timer) -> X-OR Array (determind input &
> output of 74LS574 is equal) (Vcc=+5V)
> everything goes fine until I implement a 555 timer to generate a 3Hz pulse
> (RA = RB = 1.6k ohm, C = 100 micro Farad) to trigger a 74LS574 8-bits
> D-flip-flop, when the 555 timer generated pulse, the output of my MAX4460
> will swinging same freaquency as 555 timer at 3Hz, I check the current drain
> by the circuit, when the 555 output a HIGH, the current is about 20mA, and
> LOW will be about 16mA. I did try different combination of 555 construction,
> RA = RB = 16k ohm, C=10 micro Farad, and this doesn't help. I suspected this
> meight be wheatstone bridge problem, I try to use a 4.7V zener Diode to make
> the bridge stable, and it doesn't help at all. Can somebody please help me
> to solve this problem...??? I have to done my project before Christmas......
I haven't read your entire post or thought about your problem, but I
have one comment. If you're using an actual 555 timer - one of the
original bipolar designs, they have a very intense supply-to-supply
current spike that occurs on output transitions. However none of the
improved CMOS versions have this problem. Here's a quote from the
Intersil ICM7555 data sheet,
"The ICM7555 device is, in most instances, a direct replacement for
the NE/SE555 device. However, it is possible to effect economies
in the external component count using the ICM7555. Because the
bipolar 555 device produces large crowbar currents in the output
driver, it is necessary to decouple the power supply lines with a
good capacitor close to the device. The 7555 device produces no
such transients. See Figure 1."
Figure 1 shows a horrible, nearly 400mA current spike, lasting 200us.
It's doubtful the effects of this high-current pulse on sensitive
circuitry can be simply removed with a single bypass capacitor.
Besides the ICM7555 made by Intersil, Philips and Maxim, there are
other good cmos parts, the TLC555 made by TI, the LMC555 made by NSC,
and the TS555 made by ST. Each design is slightly different.