An external reference or a DAC can be used to drive VREF over a 0V to 5V range (Figur" />
參數(shù)資料
型號: LTC1405CGN#PBF
廠商: Linear Technology
文件頁數(shù): 2/20頁
文件大小: 0K
描述: IC ADC 12BIT 5MSPS SAMPLE 28SSOP
標準包裝: 49
位數(shù): 12
采樣率(每秒): 5M
數(shù)據(jù)接口: 并聯(lián)
轉(zhuǎn)換器數(shù)目: 1
功率耗散(最大): 145mW
電壓電源: 雙 ±
工作溫度: 0°C ~ 70°C
安裝類型: 表面貼裝
封裝/外殼: 28-SSOP(0.154",3.90mm 寬)
供應商設備封裝: 28-SSOP
包裝: 管件
輸入數(shù)目和類型: 2 個單端,雙極;1 個差分,雙極
10
LTC1405
1405fa
An external reference or a DAC can be used to drive VREF
over a 0V to 5V range (Figures 3a and 3b). The input
impedance of the VREF pin is 2k, so a buffer may be
required for high accuracy. Driving VREF with a DAC is
useful in applications where the peak input signal ampli-
tude may vary. The input span of the ADC can then be
adjusted to match the peak input signal, maximizing the
signal-to-noise ratio.
Both the VCM and VREF pins must be bypassed with
capacitors to ground. For best performance, 1F or larger
ceramic capacitors are recommended. For the case of
external circuitry driving VREF, a smaller capacitor can be
used at VREF so the input range can be changed quickly. In
this case, a 0.05F or larger ceramic capacitor is accept-
able.
The VCM pin is a low output impedance 2.5V reference that
can be used by external circuitry. For single 5V supply
applications it is convenient to connect AIN– directly to the
VCM pin.
Driving the Analog Inputs
The differential inputs of the LTC1405 are easy to drive.
The inputs may be driven differentially or single-ended
(i. e., the AIN– input is held at a fixed value). The AIN– and
AIN+ inputs are simultaneously sampled and any common
mode signal is reduced by the high common mode rejec-
tion of the sample-and-hold circuit. Any common mode
input value is acceptable as long as the input pins stay
between VDD and VSS. During conversion the analog
inputs are high impedance. At the end of conversion the
inputs draw a small current spike while charging the
sample-and-hold.
For superior dynamic performance in dual supply mode,
the LTC1405 should be operated with the analog inputs
centered at ground, and in single supply mode the inputs
should be centered at 2.5V. If required, the analog inputs
can be driven differentially via a transformer. Refer to
Table 2 for a summary of the analog input and reference
configurations and their relative advantages.
VCM
SENSE
VREF
1405 F02
+
R1
10k
LOGIC
2.5V
REFERENCE
2.048V
1F
2k
TO
ADC
R2
10k
APPLICATIO S I FOR ATIO
WU
U
VOUT
VIN
LT1019A-2.5
5V
1405 F03a
1F
VREF
SENSE
5V
VCM
LTC1405
1405 F03b
1F
VREF
LTC1405
SENSE
VCM
2.048V
+
5k
LTC1450
Figure 2. Reference Circuit
Figure 3a. Using the LT1019-2.5 As an
External Reference; Input Range = ±1.25V
Figure 3b. Driving VREF with a DAC
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