參數(shù)資料
型號: LTC1749IFW#TRPBF
廠商: Linear Technology
文件頁數(shù): 6/20頁
文件大?。?/td> 0K
描述: IC ADC 12BIT 80MSPS SMPL 48TSSOP
標準包裝: 1,800
位數(shù): 12
采樣率(每秒): 80M
數(shù)據(jù)接口: 并聯(lián)
轉(zhuǎn)換器數(shù)目: 1
功率耗散(最大): 1.69W
電壓電源: 單電源
工作溫度: -40°C ~ 85°C
安裝類型: 表面貼裝
封裝/外殼: 48-TFSOP(0.240",6.10mm 寬)
供應(yīng)商設(shè)備封裝: 48-TSSOP
包裝: 帶卷 (TR)
輸入數(shù)目和類型: 2 個單端,雙極;1 個差分,雙極
14
LTC1749
1749f
can be configured in a noise gain of +2 with the addition of
two equal valued resistors between the output and invert-
ing input and between the two inputs. This however will raise
the noise contributed by the op amps.
Reference Operation
Figure 5 shows the LTC1749 equivalent reference circuitry
consisting of a 2V bandgap reference, a 3-to-1 switch, a
switch control circuit and a difference amplifier.
The 2V bandgap reference serves two functions. First, it is
assessable at the VCM pin to provide a DC bias point for
setting the common mode voltage of any external input
circuitry. Second, it is used to derive internal reference
levels that may be used to set the input range of the ADC.
An external bypass capacitor is required for the 2V refer-
ence output at the VCM pin. This provides a high frequency
low impedance path to ground for internal and external
circuitry. This is also the compensation capacitor for the
reference, which will not be stable without this capacitor.
To achieve the optimal input range for an application, the
internal reference voltage (VREF) is flexible. The reference
switch shown in Figure 5 connects VREF to one of two
internally derived reference voltages, or to an externally
derived reference voltage. The internally derived refer-
ences are selected by strapping the SENSE pin to GND for
0.7V, or to VDD for 1.125V. When 0.7V > VSENSE > 1.125V,
VSENSE is directly connected to VREF. Because of the dual
nature of the SENSE pin, driving it with a logic device is not
recommended.
Reference voltages between 0.7V and 1.125V may be
programmed with two external resistors as shown in
Figure 6a. An external reference may be used by applying
its output directly or through a resistor divider to the
SENSE pin (Figure 6b). When the SENSE pin is driven with
an externally derived reference voltage, it should be by-
passed to ground as close to the device as possible with
a 1
F ceramic capacitor.
A difference amplifier generates the high and low refer-
ences for the ADC. High speed switching circuits are
connected to these outputs and they must be externally
bypassed. Each output has two pins: REFHA and REFHB
for the high reference and REFLA and REFLB for the low
reference. The doubled output pins are needed to reduce
package inductance. Bypass capacitors must be con-
nected as shown in Figure 5.
APPLICATIO S I FOR ATIO
WU
UU
VCM
REFHA
REFLB
SENSE
TIE TO VDD FOR VREF = 1.125;
TIE TO GND FOR VREF = 0.7V;
VREF = VSENSE FOR
0.7V < VSENSE < 1.125V
2V
REFLA
REFHB
4.7
F
4.7
F
INTERNAL ADC
HIGH REFERENCE
BUFFER
VREF
0.1
F
1749 F05
LTC1749
4
DIFF AMP
1
F
1
F
0.1
F
INTERNAL ADC
LOW REFERENCE
2V BANDGAP
REFERENCE
1.125V 0.7V
RANGE
DETECT
AND
CONTROL
Figure 5. Equivalent Reference Circuit
Figure 4. Differential Drive with Op Amps
25
5V
SINGLE-ENDED
INPUT
2V
±1/2
RANGE
VCM
AIN
+
AIN
12pF
1749 F04
4.7
F
25
100
500
500
25
25
LTC1749
+
1/2 LT1818
+
1/2 LT1818
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