
LTC2258-12
LTC2257-12/LTC2256-12
18
225812fd
CONVERTER OPERATION
TheLTC2258-12/LTC2257-12/LTC2256-12arelowpower
12-bit 65Msps/40Msps/25Msps A/D converters that are
poweredbyasingle1.8Vsupply.Theanaloginputsshould
be driven differentially. The encode input can be driven
differentially or single-ended for lower power consump-
tion. The digital outputs can be CMOS, double data rate
CMOS (to halve the number of output lines), or double
data rate LVDS (to reduce digital noise in the system.)
Many additional features can be chosen by programming
the mode control registers through a serial SPI port. See
the Serial Programming Mode section.
ANALOG INPUT
The analog input is a differential CMOS sample-and-hold
circuit (Figure 2). The inputs should be driven differentially
around a common mode voltage set by the VCM output
pin, which is nominally VDD/2. For the 2V input range,
the inputs should swing from VCM – 0.5V to VCM + 0.5V.
Thereshouldbe180°phasedifferencebetweentheinputs.
CSAMPLE
3.5pF
RON
25
RON
25
VDD
LTC2258-12
AIN+
225812 F02
CSAMPLE
3.5pF
VDD
AIN–
ENC–
ENC+
1.2V
10k
1.2V
10k
CPARASITIC
1.8pF
CPARASITIC
1.8pF
10
Figure 2. Equivalent Input Circuit
applicaTions inForMaTion