Forums

LPC1768 ADC

Started by John Larkin January 8, 2020
Does anyone use the LPC176* uPs?

I need to know how much current the ADC/DAC VREFP pin uses, and I
can't find that anywhere. You'd think they might spec that.

They recommend filtering it, but the eval board uses a ferrite bead.

-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

torsdag den 9. januar 2020 kl. 02.06.09 UTC+1 skrev John Larkin:
> Does anyone use the LPC176* uPs? > > I need to know how much current the ADC/DAC VREFP pin uses, and I > can't find that anywhere. You'd think they might spec that. > > They recommend filtering it, but the eval board uses a ferrite bead. >
google says, https://community.nxp.com/thread/388939 "The input impedance of Vref is around 7k Ohms, so this should be taken into consideration when using a voltage divider on this input."
On Wed, 8 Jan 2020 17:18:35 -0800 (PST), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

>torsdag den 9. januar 2020 kl. 02.06.09 UTC+1 skrev John Larkin: >> Does anyone use the LPC176* uPs? >> >> I need to know how much current the ADC/DAC VREFP pin uses, and I >> can't find that anywhere. You'd think they might spec that. >> >> They recommend filtering it, but the eval board uses a ferrite bead. >> > >google says, https://community.nxp.com/thread/388939 > >"The input impedance of Vref is around 7k Ohms, so this should be taken into consideration when using a voltage divider on this input." > >
Thanks. I did search the NXP community posts, with various keywords, and didn't find that one. I can probably use a 50 ohm series resistor. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
John Larkin wrote...
> > I can probably use a 50 ohm series resistor.
It may need a capacitor to handle pulse currents. -- Thanks, - Win
On 8 Jan 2020 18:33:32 -0800, Winfield Hill <winfieldhill@yahoo.com>
wrote:

>John Larkin wrote... >> >> I can probably use a 50 ohm series resistor. > > It may need a capacitor to handle pulse currents.
Sure, all my voltage reference noise filters involve capacitors somewhere. Maybe I'll provide a separate voltage reference for the uP ADC, a local bandgap and cap. Then I can digitize and check the *other* reference. 3V for the uP, 2.5 for everything else. This will be a lithium niobate EO modulator driver, with a very expensive (over $200) distributed amplifier chip. It has a lot of sequencing requirements and a lot of protections. -- John Larkin Highland Technology, Inc The cork popped merrily, and Lord Peter rose to his feet. "Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
On Thu, 09 Jan 2020 08:04:22 -0800, jlarkin@highlandsniptechnology.com
wrote:

>On 8 Jan 2020 18:33:32 -0800, Winfield Hill <winfieldhill@yahoo.com> >wrote: > >>John Larkin wrote... >>> >>> I can probably use a 50 ohm series resistor. >> >> It may need a capacitor to handle pulse currents. > >Sure, all my voltage reference noise filters involve capacitors >somewhere. > >Maybe I'll provide a separate voltage reference for the uP ADC, a >local bandgap and cap. Then I can digitize and check the *other* >reference. 3V for the uP, 2.5 for everything else. > >This will be a lithium niobate EO modulator driver, with a very >expensive (over $200) distributed amplifier chip. It has a lot of >sequencing requirements and a lot of protections.
We use that series of parts. Always place some capacitance across the A/D input to analog Vss.... Usually this is my 1st order low pass anyway. For Vref+ I just use a small resistance and a capacitor. 1 Ohm or a few more does pretty well.
On Thu, 09 Jan 2020 13:58:05 -0800, boB <boB@K7IQ.com> wrote:

>On Thu, 09 Jan 2020 08:04:22 -0800, jlarkin@highlandsniptechnology.com >wrote: > >>On 8 Jan 2020 18:33:32 -0800, Winfield Hill <winfieldhill@yahoo.com> >>wrote: >> >>>John Larkin wrote... >>>> >>>> I can probably use a 50 ohm series resistor. >>> >>> It may need a capacitor to handle pulse currents. >> >>Sure, all my voltage reference noise filters involve capacitors >>somewhere. >> >>Maybe I'll provide a separate voltage reference for the uP ADC, a >>local bandgap and cap. Then I can digitize and check the *other* >>reference. 3V for the uP, 2.5 for everything else. >> >>This will be a lithium niobate EO modulator driver, with a very >>expensive (over $200) distributed amplifier chip. It has a lot of >>sequencing requirements and a lot of protections. > > >We use that series of parts. Always place some capacitance across the >A/D input to analog Vss.... Usually this is my 1st order low pass >anyway. > >For Vref+ I just use a small resistance and a capacitor. 1 Ohm or a >few more does pretty well. > >
OK, sounds like a few ohms, 10 or so maybe, and a pretty big ceramic cap would be OK. Or my dedicated 3V bandgap running at a few mA. The NXP data sheets are weird. They specify the analog power and reference currents in various shutdown modes, but not operating. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com