TSX712 used in a Sallen-Key topology: issues with some capacitors packages

[PieroMAE]

Hi everybody,

We are using TSX712 in a Sallen-key topology used as low-pass filter 

We have
C1, C2 = 1 uF
R1,R2 = 1K
Vpsupp = 5V
Vin = sinusoidal signal with amplitude 4,5 Vpp, offset=2,5V, Freq = 60 Hz

Vout is the expected one when using Capacitors package 0603inch rating-voltage = 50V ;
using other kind of Capacitors (0201inch with rating-voltage 6,3 V or 0402inch with rating-voltage = 16 V)
Vout has a distorsion in terms of an increasing gradient during increasing amplitude half-period and a decreasing gradient during decreasing amplitude half-period. So Vout sinusoidal time-offset to Vin is not constant (time-offset is decreasing during amplitude-increasing-half-period ; time-offset is increasing during amplitude-decreasing-half-period).

Vin maximum value is occurring every half period (at about 8,3 msec)
Vout maximum value is occurring before the half period (at about 7,4 msec)

This different behaviour, related to used capacitors, is expected ? There are some restrictions using 0201inch packages ? Because We need of a very small area factor.
 
 

 

 

 

(as 

observing a strange behaviour

[TDK]

In short, capacitance is a function of voltage. At 0V, the capacitance is the rated value, but it drops as the capacitor becomes charged. This effect is stronger for smaller capacitor packages. This means that your sinusoidal input will not be symmetric, even with large caps, but the effect is stronger as you move to smaller sizes.

Here's a good graph that shows the effect along with the effect of smaller packages:

Using larger caps and/or ones with larger voltage ratings is one solution. Another would be to have your input be symmetric about V=0 and bias it afterwards.

[MasterT]

Read this doc., page 5-6

 

[LCE]

For signal processing, use only NP0 / C0G ceramics (your only chance for small size), if possible with a voltage rating about 5 to 10 times higher than your expected peak-to-peak voltages. 
So it's rather the ceramics type and voltage rating than package size (which are related).

Never use ceramics called "Y" something, use the "X" types (X7R, X5R) only for buffering DC voltages.

I'm using these for years now in low noise, low distortion audio circuits.