I am puzzled by the paragraph beginning "For relatively small values of the external reference capacitors" on page 28 of the LTC2484 datasheet, and by figures 15 to 18, which suggest smaller VREF decoupling capacitors are bet ter than large ones. I would expect it to be the other way around. Is thi s a datasheet error? What am I missing?
TIA
I haven't looked at the diagrams you list but that paragraph on p28 talking about small Cref values does end by saying "... the user is advised to avoid them" ?
piglet
the external reference capacitors" on page 28 of the LTC2484 datasheet, an d by figures 15 to 18, which suggest smaller VREF decoupling capacitors are better than large ones. I would expect it to be the other way around. Is this a datasheet error? What am I missing?
I think the error comes in that last sentence of that second paragraph. Ju st replacing the word "Such" with "Large" would fix it.
I made the below LTSpice sim. It shows that large CREF causes FS errors, a s figures 15-18 indicate. For small CREF, although the voltage on the VREF pin dips, it quickly and fully recovers. For large CREF, the sampling cur rent (I=CVf) is smoothed-out, producing a DC drop across Rsource.
Version 4 SHEET 1 1444 680 WIRE -96 112 -176 112 WIRE 48 112 -16 112 WIRE 176 112 48 112 WIRE 288 112 176 112 WIRE 544 112 368 112 WIRE 608 112 544 112 WIRE 736 112 608 112 WIRE 912 112 816 112 WIRE -176 128 -176 112 WIRE 48 128 48 112 WIRE 544 128 544 112 WIRE 352 208 352 160 WIRE 800 208 800 160 WIRE -176 224 -176 208 WIRE 48 224 48 192 WIRE 544 224 544 192 WIRE 912 224 912 112 WIRE 304 304 304 160 WIRE 352 304 352 288 WIRE 752 304 752 160 WIRE 800 304 800 288 FLAG 48 224 0 FLAG 544 224 0 FLAG 912 224 0 FLAG -176 224 0 FLAG 304 304 0 FLAG 752 304 0 FLAG 352 304 0 FLAG 800 304 0 FLAG 176 112 VREF FLAG 608 112 VSAMPLE SYMBOL sw 272 112 R270 SYMATTR InstName S1 SYMATTR Value SW1 SYMBOL sw 720 112 R270 SYMATTR InstName S2 SYMATTR Value SW2 SYMBOL cap 528 128 R0 SYMATTR InstName Csample SYMATTR Value 10p SYMBOL cap 32 128 R0 SYMATTR InstName Cexternal SYMATTR Value {CREF} SYMBOL res 0 96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName Rsource SYMATTR Value 100 SYMBOL voltage -176 112 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 1 SYMBOL voltage 352 192 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value PULSE(-1 1 0 1p 1p 625n 1u) SYMBOL voltage 800 192 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V3 SYMATTR Value PULSE(-1 1 800n 1p 1p 100n 1u) TEXT -248 304 Left 2 !.model SW1 SW(Ron=10k, Roff=9e9, Vt=0) TEXT 888 328 Left 2 !.tran 0 9m99979 9m999 TEXT 368 344 Left 2 !.step param CREF list 100p 1n 10n TEXT -248 344 Left 2 !.model SW2 SW(Ron=1, Roff=9e9, Vt=0)
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