参数资料
型号: LTC2221CUP#TRPBF
厂商: Linear Technology
文件页数: 15/32页
文件大小: 0K
描述: IC ADC 12-BIT 135MSPS 64-QFN
标准包装: 2,000
位数: 12
采样率(每秒): 135M
数据接口: 并联
转换器数目: 1
功率耗散(最大): 931mW
电压电源: 单电源
工作温度: 0°C ~ 70°C
安装类型: 表面贴装
封装/外壳: 64-WFQFN 裸露焊盘
供应商设备封装: 64-QFN(9x9)
包装: 带卷 (TR)
输入数目和类型: 1 个单端,双极; 1 个差分,双极
LTC2220/LTC2221
22
22201fa
Other voltage ranges in between the pin selectable ranges
can be programmed with two external resistors as shown
in Figure 10. An external reference can be used by applying
its output directly or through a resistor divider to SENSE.
It is not recommended to drive the SENSE pin with a logic
device. The SENSE pin should be tied to the appropriate
level as close to the converter as possible. If the SENSE pin
is driven externally, it should be bypassed to ground as
close to the device as possible with a 1
F ceramic capacitor.
2. Use as large an amplitude as possible; if transformer
coupled use a higher turns ratio to increase the amplitude.
3. If the ADC is clocked with a sinusoidal signal, filter the
encode signal to reduce wideband noise.
4. Balance the capacitance and series resistance at both
encode inputs so that any coupled noise will appear at both
inputs as common mode noise. The encode inputs have a
common mode range of 1.1V to 2.5V. Each input may be
driven from ground to VDD for single-ended drive.
APPLICATIO S I FOR ATIO
WU
UU
VDD
LTC2220/LTC2221
22201 F11
VDD
ENC–
ENC+
1.6V BIAS
1:4
0.1
F
CLOCK
INPUT
50
6k
TO INTERNAL
ADC CIRCUITS
Figure 11. Transformer Driven ENC+/ENC
VCM
SENSE
1.6V
0.8V
2.2
F
12k
1
F
12k
22201 F10
LTC2220/
LTC2221
Figure 10. 1.6V Range ADC
Input Range
The input range can be set based on the application. The
2V input range will provide the best signal-to-noise perfor-
mance while maintaining excellent SFDR. The 1V input
range will have better SFDR performance, but the SNR will
degrade by 5dB. See the Typical Performance Character-
istics section.
Driving the Encode Inputs
The noise performance of the LTC2220/LTC2221 can
depend on the encode signal quality as much as on the
analog input. The ENC+/ENCinputs are intended to be
driven differentially, primarily for noise immunity from
common mode noise sources. Each input is biased through
a 6k resistor to a 1.6V bias. The bias resistors set the DC
operating point for transformer coupled drive circuits and
can set the logic threshold for single-ended drive circuits.
Any noise present on the encode signal will result in
additional aperture jitter that will be RMS summed with the
inherent ADC aperture jitter.
In applications where jitter is critical (high input frequen-
cies) take the following into consideration:
1. Differential drive should be used.
Maximum and Minimum Encode Rates
The maximum encode rate for the LTC2220/LTC2221 is
170Msps (LTC2220) and 135Msps (LTC2221). For the
ADC to operate properly, the encode signal should have a
50% (
±5%) duty cycle. Each half cycle must have at least
2.8ns (LTC2220) or 3.5ns (LTC2221) for the ADC internal
circuitry to have enough settling time for proper operation.
Achieving a precise 50% duty cycle is easy with differential
sinusoidal drive using a transformer or using symmetric
differential logic such as PECL or LVDS.
An optional clock duty cycle stabilizer circuit can be used
if the input clock has a non 50% duty cycle. This circuit
uses the rising edge of the ENC+ pin to sample the analog
input. The falling edge of ENC+ is ignored and the internal
falling edge is generated by a phase-locked loop. The input
clock duty cycle can vary from 30% to 70% and the clock
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