Capacitor notation in datasheet

Thread Starter

salvus

Joined Apr 2, 2020
16
Hey, I am reading a datasheet and it shows a circuit with some electrolytic capacitors. Page 18 of this https://www.ti.com/lit/ds/snosbq4e/...199&ref_url=https%3A%2F%2Fwww.google.co.uk%2F

what does it mean when it just provides a decimal number. What unit is it? For one of the capacitors it says uF.

As a separate question. In this circuit some of the capacitors connect directly to ground. To save space and the number of components needed, could I just use a single capacitor that connects to ground and have all those pins connect to it instead of separate capacitors?

many thanks
 

Yaakov

Joined Jan 27, 2019
3,161
Hey, I am reading a datasheet and it shows a circuit with some electrolytic capacitors. Page 18 of this https://www.ti.com/lit/ds/snosbq4e/snosbq4e.pdf?ts=1618582583199&ref_url=https%3A%2F%2Fwww.google.co.uk%2F

what does it mean when it just provides a decimal number. What unit is it? For one of the capacitors it says uF.

As a separate question. In this circuit some of the capacitors connect directly to ground. To save space and the number of components needed, could I just use a single capacitor that connects to ground and have all those pins connect to it instead of separate capacitors?

many thanks
EDIT! I mistyped, thanks to @Ian0 for spotting it.
mF μF, no—they are all different values. The do all connect to ground but each connects to a different pin.

1618583949640.png
 
Last edited:

vanergr

Joined Nov 23, 2020
9
As already stated the capacitors are connected to different pins but not only that some of them serve different purpose, e.g. the cap at Pin 3 is for DC blocking while the 1uF at Pin4 is for AC/noise decupling. In addition it is better to use multiple values of capacitance to decouple AC signals because it helps with different frequencies that can be present.
 

Thread Starter

salvus

Joined Apr 2, 2020
16
Many thanks. I'm starting to get it I think. It seems to be very temperamental. I think it's down to my lack of understanding of what the capacitors are really doing. I'm finding the datasheet quite hard to follow. Through experimentation, I can see how R1 and C1 set the frequency. When I use a potentiometer I am able to set it with reasonable accuracy against what the equation on page 14 says it will be.

My understanding is that C2 controls the bandwidth. There are two different equations to show what the bandwidth will be depending on if the input voltage is less than or greater than 200mVrms. When I use smaller value capacitors like shown in the example in the datasheet I get lots of false positives across the frequency spectrum. When I use 1uF on C2 it seems to be stable. From this, I have chosen C3 to be 3.3uF because that is the closest I had on hand to satisfy the suggested rule of C3 > 2C2.

What does the capacitor connected to Pin 4 and GND do? I am not seeing that it makes a difference.

Thanks for your help :)
 

Yaakov

Joined Jan 27, 2019
3,161
Power supply decoupling capacitor
Yes, it decouples the power supply from the chip to reduce the variations in Vcc the chip sees. It deals with sudden demands of the chip for power so the inductance in the traces doesn't cause a lag in supplying the current needed.

If you looked at the output waveform on a scope, you'd see (more) distortion without the cap in place.
 
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