LTC2194_15 Datasheet, PDF (17/28 Pages) Linear Technology – 16-Bit, 125/105/80Msps Low Power Dual ADCs

LTC2195
LTC2194/LTC2193
APPLICATIONS INFORMATION
CONVERTER OPERATION
The LTC2195/LTC2194/LTC2193 are low power, 2-channel,
16-bit, 125/105/80Msps A/D converters that are powered
by a single 1.8V supply. The analog inputs should be driven
differentially. The encode input can be driven differentially
or single ended for lower power consumption. To minimize
the number of data lines the digital outputs are serial LVDS.
Each channel outputs two bits at a time (2-lane mode) or
four bits at a time (4‑lane mode). At lower sampling rates
there is a one bit per channel option (1-lane mode). Many
additional features can be chosen by programming the
mode control registers through a serial SPI port.
ANALOG INPUT
The analog inputs are differential CMOS sample-and-
hold circuits (Figure 2). The inputs should be driven
differentially around a common mode voltage set by the
V_CM1 or V_CM2 output pins, which are nominally V_DD/2.
For the 2V input range, the inputs should swing from
V_CM – 0.5V to V_CM + 0.5V. There should be 180° phase
difference between the inputs.
The two channels are simultaneously sampled by a shared
encode circuit (Figure 2).
Single-Ended Input
For applications less sensitive to harmonic distortion, the
A_IN+ input can be driven singled ended with a 1V_P-P signal
centered around V_CM. The A_IN– input should be connected
to V_CM and the V_CM bypass capacitor should be increased
to 2.2µF. With a singled-ended input the harmonic distortion
and INL will degrade, but the noise and DNL will remain
unchanged.
INPUT DRIVE CIRCUITS
Input Filtering
If possible, there should be an RC lowpass filter right at
the analog inputs. This lowpass filter isolates the drive
circuitry from the A/D sample-and-hold switching, and
also limits wideband noise from the drive circuitry. Figure  3
shows an example of an input RC filter. The RC component
values should be chosen based on the application’s input
frequency.
Transformer Coupled Circuits
Figure 3 shows the analog input being driven by an RF
transformer with a center-tapped secondary. The center
LTC2195
V_DD
10Ω
A_IN+
V_DD
10Ω
A_IN–
V_DD
C_PARASITIC
1.8pF
C_PARASITIC
1.8pF
R_ON
C_SAMPLE
5pF
15Ω
C_SAMPLE
R_{ON 5pF}
15Ω
ENC⁺
1.2V
10k
ENC^–
10k
1.2V
219543 F02
Figure 2. Equivalent Input Circuit. Only One of Two Analog Channels is Shown
219543f
17