Hello branadic,
it´s often good too ask some questions.
There is a risk for getting blind for the things I do as a habit with my ADC´s.
This week someone
asked me about the discrepancy of the decoupling capacitor for the LTC2400 and the LT1027.
In LTC2400 datasheet a 100nF ceramics and a 10uF Ta is recommended.
In LT1027 datasheet either a 4.7 uF or no decoupling capacitor at the VREF output is recommended.
The LT1027 datasheet tells that with a 100nF capacitor the output ringing will be a maximum.
In my cirquits I use 100nF and 10uF as a habit (for critical debouncings).
The intention here is that the 10uF Ta with its ESR of around 2 Ohms acts similar to a snubber network (Boucherot)
to dampen any oscillations created by a ceramics capacitor (with wire inductivity).
But after the question I was not shure: will this work in any case and also with the LT1027?
-> I had to check this.
I tested 3 different configurations together with ADC24 (slotted PCB).
VREF1: 100nF X7R + 10uF Ta Size A (initial configuration)
VREF2: 10uF Ta alone
VREF3: 100nF X7R
VREF4: same as VREF1 (after all changes)
The 10uF Ta is a RS-components 684-4443
http://de.rs-online.com/web/p/tantal-elektrolytkondensator/6844443/I measured VREF with a 200 MHz BW (+10:1 probe with 250 MHz BW)
oscilloscope with short connection to the D-Sub connector (see photo)
to see any oscillations of the reference.
In parallel I measured LTZ#4 via 2:1 divider with the ADC.
First surprise: there are large spikes (200-300 mVpp) with risetimes in the 2-3 ns region
(near the BW-limit of the scope = 2 ns) from the internal 153 kHz clock of the LTC2400.
This is something that I never expected from a analog cirquit.
To the scope pictures:
Overview shows the first 5 ms of the conversion.
There is a large first spike (start of conversion) followed by some smaller spikes
on each edge of the 153 kHz clock (mostly switching noise of VREF input).
Zoom 500x10 (x = 500 y = 10) shows the conversion clock.
VREF3 (100nF alone) shows large oscillations between the clock.
The other configurations do not differ much.
Zoom 10k (x = 10000) shows a single edge of the conversion clock.
The ringing is for the 10uF and 10uF + 100nF configuration
shorter than the 460ns datasheet spec.
So from the scope pictures there is not much difference between the
10uF and the 10uF + 100nF configuration.
The maximum spike amplitudes seem to be 3-5% smaller with the
combination of the capacitors.
On the 100nF alone configuration the ringing is clearly visible.
(see also table).
The comparison of the ADC24 readings shows a factor 2-3 increased uVpp noise for the 100nF alone capacitor.
The readings of the LTZ#3 are also increased by 170 uV (=47ppm) in VREF3 against the VREF1 configuration.
But also the VREF2 configuration (10 uF alone) has a increased reading of 7.5 ppm against VREF1 configuration.
So which one is nearer on the truth?
This gets clearer when measuring the own 5V reference through the 2:1 divider on the ADC input.
In this case the theoretical reading should be exactly 2500 mV.
In practice there is some loss in the 2:1 divider (which can be seen by the HP34401A high impedance measurement).
The actual value is around 20uV or 7.6 ppm lower than the half value.
So the nominal reading in this case should be 2499.981 mV.
VREF2 is +9.7 ppm too high
VREF3 +30.5 ppm and
VREF4 is -3.5ppm (low)
I not really thought that the decoupling capacitor influences the readings (INL)
that much regarding the dynamic resistance of the VREF in the mOhm range.
So there is shurely room for some improvement by the right placement and value of the decoupling capacitors.
with best regards
Andreas
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