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I was just looking at the schematic shown below for a DAC. Anyone know why the iron core inductor (L1 circled in red) is included? My guess is that it's used to ramp the power up/down when powering up/down the unit to help prevent the common "popping" sound that occurs in speakers, but I'm far from certain of that.

Additionally, I'm confused by the units of the inductor. "2500" what? Anyone know?

enter image description here

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It is a power supply decoupling inductor. Without an accompanying circuit theory document from the designer, I could not tell you why it was selected or even exactly what that 2500 means.

Normally if there is a number next to an inductor it is the inductance in micro Henrys. On the one hand, a 2500 micro Henry inductor is going to be pretty big. On the other hand, if they are trying to suppress audio frequency noise it might make sense.

If it were me, I would try to find a bill of materials, or a picture of the board. The first would tell you for sure, with a picture, you could look at the size of the inductor at least make a guess.

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Based on the picture, it looks like a ferrite bead.

Ferrite beads are rated at impedance at 100MHz.

Ferrite beads with impedance of 2500 ohms at 100MHz do exist.

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Doesn't look like an iron core in such a small package (603?) with 2.5 mH so it must be 2500 nH.

It's hard to tell why they used an inductor because the LPF BW is so high, much higher than the max Nyquist rate of the ADC. But I suspect they use a > 1 MHz 3.3V SMPS which caused some aliasing PSRR issues. But that's a SWAG !

enter image description here

Notice that the Inductor needs to be small to get a high resistance, found in tiny 805 2.5uH parts like 3.8 Ohms to get near critically damped. However this looks like a 603 part.

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That specific arrangement is called a pi filter. It acts as a low pass:

enter image description here

In this case, the 2500 is in units of nH, and the filter applies 10dB attenuation at ~200 KHz. From the shape of the transfer function, the designer was concerned about rejecting switch mode power supply noise, which would be centered somewhere around 500-1000 KHz.

Depending on what your power supply is, you might choose to change those filter values, or even omit the filter entirely. I might at least try to damp down the resonant peak at 95 KHz, which could be a problem unless your supply is very quiet at that frequency. If you were more concerned about low frequency noise (for example, 60 Hz hum which will not be attenuated at all by this filter), you might even replace it with a linear regulator.

Edit: If you look at the revised circuit, seems they added in damping to remove that resonant peak:

revised circuit

Although now the attenuation is much lower, which makes me wonder if the filter is even needed.

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  • \$\begingroup\$ You meant 400kHz, but wrote 400Hz. At least I hope so -- because I 'fixed' it. \$\endgroup\$
    – TimWescott
    Apr 12 at 0:56
  • \$\begingroup\$ @TonyStewartEE75 I made the plot quickly using an existing SPICE model I had for a different filter and didn't bother to change it. Seems I was mistaken about how the AC analysis works, I'll fix it \$\endgroup\$ Apr 12 at 1:01
  • \$\begingroup\$ At least you corrected the 2.5mH it's not possible in a 603 part \$\endgroup\$ Apr 12 at 1:10
  • \$\begingroup\$ @TonyStewartEE75 The response I got was so reasonable I assumed I had the supply right. I'm surprised how bad the resonant peaking is in the corrected circuit. \$\endgroup\$ Apr 12 at 1:18

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