参数资料
型号: LTC6405CMS8E#TRPBF
厂商: LINEAR TECHNOLOGY CORP
元件分类: 模拟信号调理
英文描述: SPECIALTY ANALOG CIRCUIT, PDSO8
封装: LEAD FREE, MSOP-8
文件页数: 8/24页
文件大小: 281K
代理商: LTC6405CMS8E#TRPBF
LTC6405
16
6405fa
APPLICATIONS INFORMATION
In general, the degree of feedback pair mismatch is a
source of common mode to differential conversion of both
signals and noise. Using 1% resistors or better will mitigate
most problems, and will provide about 34dB worst case of
common mode rejection. Using 0.1% resistors will provide
about 54dB of common mode rejection. A low impedance
ground plane should be used as a reference for both the
input signal source and the VOCM pin. Bypassing the VOCM
with a high quality 0.1μF ceramic capacitor to this ground
plane will further help prevent common mode signals from
being converted to differential signals.
There may be concern on how feedback factor mismatch
affects distortion. Feedback factor mismatch from using
1% resistors or better, has a negligible effect on distortion.
However, in single supply level shifting applications where
there is a voltage difference between the input common
mode voltage and the output common mode voltage,
resistor mismatch can make the apparent voltage offset
of the amplier appear worse than specied.
The apparent input referred offset induced by feedback
factor mismatch is derived from the above equation:
VOSDIFF(APPARENT) ≈ (VICM – VOCM) Δβ
Using the LTC6405 in a single supply application on a
single 5V supply with 1% resistors, and the input com-
mon mode grounded, with the VOCM pin biased at 2.5V, the
worst case DC offset can induce 25mV of apparent offset
voltage. With 0.1% resistors, the worst case apparent
offset reduces to 2.5mV.
Input Impedance and Loading Effects
The input impedance looking into the VINP or VINM input
of Figure 1 depends on whether or not the sources VINP
and VINM are fully differential or not. For balanced input
sources (VINP = –VINM), the input impedance seen at either
input is simply:
RINP = RINM = RI
For single ended inputs, because of the signal imbalance
at the input, the input impedance actually increases over
the balanced differential case. The input impedance looking
into either input is:
R
INP = RINM =
R
I
1–
1
2
R
F
R
I + RF
Input signal sources with non-zero output impedances
can also cause feedback imbalance between the pair of
feedback networks. For the best performance, it is rec-
ommended that the input source output impedance be
compensated for. If input impedance matching is required
by the source, a termination resistor R1 should be chosen
(see Figure 4):
R1
=
R
INM RS
R
INM –RS
Figure 4. Optimal Compensation for Signal Source Impedance
VS
+
+
RF
RI
RINM
RS
RI
R2 = RS || R1
R1 CHOSEN SO THAT R1 || RINM = RS
R2 CHOSEN TO BALANCE R1 || RS
R1
6405 F04
According to Figure 4, the input impedance looking into
the differential amp (RINM) reects the single ended source
case, thus:
R
INM =
R
I
1–
1
2
R
F
R
I + RF
R2 is chosen to equal R1 || RS:
R2
=
R1 R
S
R1
+R
S
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