#oggstreamer – measuring power supply and input amplifier

Today I noticed that I can’t put every project progress on the Blog – so I started to use the WIKI more. Nevertheless – in my opinion – it is really important to check the power supply rails of your design. So I decided to put this on the Blog. I measured the power lines and all the corresponding ripple voltages – and it looks quite good. I measured the ripple by using the “AC Coupling”-Mode on the oscilloscope. And I added the pictures together – so in the pictures on top you’ll see “DC”-View and on the bottom the “AC”-View. I also used some kind of precision probe (a salvaged spring from an old printer) – to keep the ground line short – so that I don’t measure something else than the actual ripple: (sorry for the bad foto quality but I only have my cell phone at hand at the moment)

All the measurements were done while the OggStreamer was connected to a PC Sound card playing 440Hz with an amplitude of about 0,5V-PP and ogg encoding enabled.

• 5V Main-Power Supply measured @ C78:

I am using a 5V-wall power supply from Schukat (Order Number: HNP06-050) Click here for the Datasheet. Specs say typical 100mV-PP ripple. I measured about 50mV-PP ripple.

• 5V OPV-Supply measuered @ C18

The measured ripple is around 32mV which is not to much. Also considering the the used OPA LME49721 has a excellent Power Supply Ripple Rejection (PSRR) of 103 dB. The measured ripple has a frequency from about 45 Hz – I don’t know were this comes from – maybe it is the main line – but anyhow the ripple is  negligible.

• 3V3 Digital Supply measured @ C56

Everything is fine here – Ripple is about 25mV-PP

• 1V8 Digital Supply measured @ C71

Everything is fine here – Ripple is very low around 6mV-PP

• 3V3 Analog Supply measured @ C10

The VS1063 has uses a separate Analogue Supply  Voltage. It could be tied together with the 3V3 digital supply, but in our design we decided to seperate those two rails be an small coil to block high frequency noise. The pcb layout guidlines for  VS10xx are stating, that you should tie them together directly – So maybe the coil ferrite bead (I got confused and mixed coils and ferrite beads up) will be replaced by a zero-Ohm bridge. The VS1063 is said to have a decent PSRR as well, although I can’t find the numbers in the Datasheet. The measured 35mV-PP should be OK.

Next I also measured the input amplifier. The input amplifier for the OggStreamer is an inverting OPA designed to be capable of amplifying or to attenuate the signal as needed. The gain is designed to range from 0.1 to 11, which translates to about plus/minus 20dB.

• 440Hz Signal measured before the OPA @ C25 (left) and C32 (right)

Left is 286mV-PP and Right is 288mV-PP

• 440Hz Signal measured after the OPA – in amplifier mode (x10) @ C20 (left) and C4 (right)

Left is 2.5V-PP and Right is 2.56mV-PP

• 440Hz Signal measured after the OPA – in attenuator mode (x0.1) @ C20 (left) and C4 (right)

Left is 30mV-PP and Right is 32mV-PP

It seems like the output of the PC-Soundcard is slightly unbalanced. Doing the math the OPA circuit has a gain from 0,105 to 8,74 – which is almost 0,1 – 11 as designed. I want to note that  I am currently using standard 0805 Resistors for the OPA, which will be replaced by Mini-Melfs metal-film resistors in future. The values for the Restors are 1k for R13 and R35 and 10k for R7 and R30 – this means that the Input load is about 1k.