LTC2164_15 Datasheet, PDF (19/36 Pages) Linear Technology – 16-Bit, 125/105/80Msps Low Power ADCs

LTC2165/LTC2164/LTC2163
Applications Information
CONVERTER OPERATION
The LTC2165/LTC2164/LTC2163 are low power, 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. 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.
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_CM output
pin, which is 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.
Single-Ended Input
For applications less sensitive to harmonic distortion, the A_IN+
input can be driven
around V_CM. The A
s_INin_– ginlep-uetnsdheoduwlditbheac1oVn_{Pn-ePcstiegdntaol} cVe_Cn_Mtearnedd
LTC2165
V_DD
10Ω
A_IN+
V_DD
A_IN–
10Ω
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
2165 F02
Figure 2. Equivalent Input Circuit
the V_CM bypass capacitor should be increased to 2.2µF.
With a single-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
tap is biased with V_CM, setting the A/D input at its optimal
DC level. At higher input frequencies a transmission line
balun transformer (Figures 4 through 6) has better bal-
ance, resulting in lower A/D distortion.
Amplifier Circuits
Figure 7 shows the analog input being driven by a high
speed differential amplifier. The output of the amplifier is
AC coupled to the A/D so the amplifier’s output common
mode voltage can be optimally set to minimize distortion.
At very high frequencies an RF gain block will often have
lower distortion than a differential amplifier. If the gain
block is single-ended, then a transformer circuit (Figures 4
through 6) should convert the signal to differential before
driving the A/D.
50Ω
V_CM
ANALOG
INPUT
0.1µF T1
1:1
25Ω
25Ω 0.1µF
25Ω _25Ω
0.1µF
A_IN+
12pF
A_IN–
LTC2165
T1: MA/COM MABAES0060 RESISTORS,
CAPACITORS ARE 0402 PACKAGE SIZE
2165 F03
Figure 3. Analog Input Circuit Using a Transformer.
Recommended for Input Frequencies from 5MHz to 70MHz
216543f
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