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TDA1543 filter capacitor calculation
TDA1543 filter capacitor calculation
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Old 16th February 2019, 06:42 PM   #1
gaszto is offline gaszto  Hungary
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Default TDA1543 filter capacitor calculation

Dear forum members,

Could you somebody explain please, how can I calculate the capacitance of the marked elements? It's not clearly visible what is above the capacitors. I see LPF, 49kHz (this sould be a cutoff freq., the 6dB/octave, and 1.2nF*8?)
What is the relation between its capacitance and the 680o Rout resistors? Is there any affect of the number of the DAC-IC? The capacitance should be different if I want to use 1 or 2 DAC-IC, instead of 4?

Thanks
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Old 16th February 2019, 08:46 PM   #2
MarcelvdG is offline MarcelvdG  Netherlands
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Neglecting the de-emphasis circuit and regarding the DC blocking caps as short circuits for signal frequencies:

On each line you have four paralleled 1.2 nF capacitors, 680 ohm, 47 kohm and whatever the resistance of the circuit connected to the outputs may be. Assuming that last one to be 10 kohm (guess), the cut-off frequency is:

f = 1/(2 pi RC) = 1/(2 pi (680 ohm // 47 kohm // 10 kohm)*(4 * 1.2 nF) ~= 52782 Hz

where // means the value of the parallel connection, so R1 // R2 // R3 = 1/(1/R1 + 1/R2 + 1/R3).

Assuming infinite input impedance of the next stage:

f = 1/(2 pi RC) = 1/(2 pi (680 ohm // 47 kohm)*(4 * 1.2 nF) ~= 49466 Hz

With fewer DAC ICs, you can either keep the resistances and the capacitances on each line the same (that is, keep 4.8 nF total from each line to ground) and accept that the output signal gets smaller, or you can scale up all resistances and scale down the filter capacitances. That is, use twice the resistance and half the capacitance with two DAC chips.
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Old 16th February 2019, 09:51 PM   #3
gaszto is offline gaszto  Hungary
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Thanks for the explanation
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Old 17th February 2019, 10:34 PM   #4
gaszto is offline gaszto  Hungary
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One more question about the DAC. I have attached a picture from the frequency response of my DAC, how can I improve the high freqs.? At 20kHz -4dB, and 18kHz -3dB.
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Old 18th February 2019, 02:15 AM   #5
abraxalito is offline abraxalito  United Kingdom
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You could try using a CLC filter between your DAC and your I/V resistors. This can be tuned to give a rising response (reaching +3.2dB @ 20kHz) before rolling off to reduce the energy of images.
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Old 18th February 2019, 10:21 AM   #6
gaszto is offline gaszto  Hungary
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Could you help to suggest a design tool or expressions to calculate the correct values? Sorry but I'm mechanical engineer, so I don't have a deep knowledge about the electronics, but I want to learn it
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Old 18th February 2019, 10:21 AM   #7
DF96 is offline DF96  England
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The drooping HF frequency response is to be expected from a NOS DAC with a crude output filter (or no output filter). Is that what you have?
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Old 18th February 2019, 10:57 AM   #8
abraxalito is offline abraxalito  United Kingdom
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Here's a starting point - you can download LTSpice (from Analog Devices) and tweak it to your heart's content.

The inductor's a Coilcraft MSS1210 and the caps I've used in such circuits have been SMT NP0 types (good stability).
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Old 18th February 2019, 11:24 AM   #9
gaszto is offline gaszto  Hungary
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Thanks DF96, Yes I only have the RC filter, same as Kusunoki-DAC.
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Old 18th February 2019, 11:25 AM   #10
gaszto is offline gaszto  Hungary
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Ok, I try to adjust the Ltspice model.
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