problems with USB power when using ADC 12bit in JM Family

[Betadine]

Hi all,

3 weeks ago I posted questions about which Micro use for 12 bit ADC conversions. I chose JM family and we spoke about the problems I would found in 12bit conversions, specially Mac who post some design issues steps:

Well! Im running with a JMDemo Board, 12bit ADC, and reading the values in Excel, I take an average of 50 values and it works fine.

Problem: Instability in Vcc.

There is no noise in the data signals, all works better than good, but the problem is that I take the power from the USB, and when I looked with the Multimeter I realized why the values moves a little in the ADC. Is the USB Power!

So, Data acquisition units that take the power from the USB... what they do? Can anyone shed some light in this?

The goal is to stabilize the 5V USB Power signal, but I dont know any common method to do it.

Do I need to reduce to around 3V to stabilize it? (cause the stabilizer chip)

What do you think?

Regards

Cristian.

[dschwertb1]

Hi!

Are you aware that the JM60 has a bandgap voltage reference on board?

Unfortunately, this can' t be used as reference for the ADC, but you may be able to use a sequence like that:

 - read bandgap

 - read signal

 - read bandgap

From the bandgap reading, you can calculate the current supply/reference voltage.

If it does not change too fast, you may get the required accuracy from this approach.

(Having a bandgap but not being able to use it as ADC reference directly is something that I never understood...)

Regards,

Dietmar

[Betadine]

Thanks for the tip, but I use the Demo Board for a quick prototype to see if it works, then I will create an embedded board avoiding the Demo. Even that, the ADR444BRZ has 1,6mv tolerance (including temperature, etc.. ) . Dont know nothing about that you say, I will try to read something about that if I encounter that, but I have already bought the chip and I will tell you how it works in a few hours

[Betadine · Accepted solution]

Well, it seems that a voltage reference fit this need.

Specially the ADR444BRZ from analog devices, Vout = 4096V with an impressive max tolerance of 1.6mv totally independent of temperature.

DS speaks about using this in PCMCIA and data adquisition units with ADCs.

Cheers!

[bigmac]

Hello Cristian,

You are correct about using an external voltage reference to provide the required reference stability.  However, check that its minimum supply voltage, assuming a three-terminal device, is less than the minimum Vdd for the MCU, presumably about 4.5 volts.  You should also take into account any noise fluctuations that may occur on the Vdd line.  If the reference drop out voltage is too high, a lower reference voltage might need to be chosen.  The minimum allowable is 2.7 volt.

Regards,

Mac

[Betadine]

Yes, the chip I have posted has

Vin: 4.5 - 12 volts

Vout: 4V

USB power: 5V

So the reference voltage is only applied to pin VrefADC of the micro, the rest take the power directly from the USB.

It will work then with 0-4V for the ADC... it can be tweaked in the source. It must be work by that way.

The rest of the signals has no noise.

USB Power must be higher than 4.5V... that´s the only dangerous requirement, I think it will work... well "it should work"! jeje. Hope not to encounter any trouble with it.

All the Best.

Cristian