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LTC6403HUD-1#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
LTC6403-1
13
64031fa
APPLICATIONS INFORMATION
When the feedback ratios mismatch (Δβ), common mode
to differential conversion occurs.
Setting the differential input to zero (V
INDIFF
= 0), the de-
gree of common mode to differential conversion is given
by the equation:
VVVVV
OUTDIFF OUT OUT INCM OCM
AVG
=≈
()
Δ
+
––•
–
β
β
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 V
OCM
pin. Directly
shorting V
OCM
to this ground or bypassing the V
OCM
with
a high quality 0.1μF ceramic capacitor to this ground plane
will further mitigate against common mode signals being
converted to differential.
There may be concern on how feedback ratio mismatch
affects distortion. Distortion caused by feedback ratio mis-
match using 1% resistors or better is negligible. 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
amplifi er appear worse than specifi ed.
The apparent input referred offset induced by feedback
ratio mismatch is derived from the above equation:
V
OSDIFF(APPARENT)
≈ (V
INCM
– V
OCM
) • Δβ
Using the LTC6403-1 in a single supply application on a
single 5V supply with 1% resistors, and the input common
mode grounded, with the V
OCM
pin biased at mid-supply,
the worst case mismatch can induce 25mV of apparent
offset voltage. With 0.1% resistors, the worst case appar-
ent offset reduces to 2.5mV.
Input Impedance and Loading Effects
The input impedance looking into the V
INP
or V
INM
input
of Figure 1 depends on whether the sources V
INP
and
V
INM
are fully differential. For balanced input sources
(V
INP
= –V
INM
), the input impedance seen at either input
is simply:
R
INP
= R
INM
= R
I
For single ended inputs, because of the signal imbalance
at the input, the input impedance increases over the bal-
anced differential case. The input impedance looking into
either input is:
RR
R
R
RR
INP INM
I
F
IF
==
+
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
1
1
2
–•
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 recommended
that the source’s output impedance be compensated. If
input impedance matching is required by the source, R1
should be chosen (see Figure 4):
R
RR
RR
INM S
INM S
1
=
•
–
According to Figure 4, the input impedance looking into
the differential amp (R
INM
) refl ects the single ended source
case, thus:
R
R
R
RR
INM
I
F
IF
=
+
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
1
1
2
–•
R2 is chosen to balance R1 || RS:
R
RR
RR
IS
IS
2
=
+
•
LTC6403-1
14
64031fa
APPLICATIONS INFORMATION
Input Common Mode Voltage Range
The LTC6403-1’s input common mode voltage (V
ICM
) is
defi ned as the average of the two input voltages, V
+IN
, and
V
–IN
. It extends from V
–
to 1.4V below V
+
.
For fully differential input applications, where V
INP
= –V
INM
,
the input common mode voltage is approximately (Refer
to Figure 5):
V
VV
V
R
RR
V
R
R
ICM
IN IN
VOCM
I
IF
CM
F
=
+
≈
+
⎛
⎝
⎜
⎞
⎠
⎟
+
+ –
•
•
2
FFI
R+
⎛
⎝
⎜
⎞
⎠
⎟
With singled ended inputs, there is an input signal com-
ponent to the input common mode voltage. Applying only
V
INP
(setting V
INM
to zero), the input common voltage is
approximately:
V
VV
V
R
RR
V
R
R
ICM
IN IN
VOCM
I
IF
CM
F
=
+
≈
+
⎛
⎝
⎜
⎞
⎠
⎟
+
+ –
•
•
2
FFI
INP F
FI
R
VR
RR+
⎛
⎝
⎜
⎞
⎠
⎟
+
+
⎛
⎝
⎜
⎞
⎠
⎟
2
•
Output Common Mode Voltage Range
The output common mode voltage is defi ned as the aver-
age of the two outputs:
VV
VV
OUTCM VOCM
OUT OUT
==
+
+ –
2
The V
OCM
pin sets this average by an internal common
mode feedback loop. The output common mode range
extends from 1.1V above V
–
to 1V below V
+
. The V
OCM
pin sits in the middle of an internal voltage divider which
sets the default mid-supply open circuit potential.
In single supply applications, where the LTC6403-1 is
used to interface to an ADC, the optimal common mode
input range to the ADC is often determined by the ADC’s
reference. If the ADC makes a reference available for set-
ting the input common mode voltage, it can be directly
tied to the V
OCM
pin, but must be capable of driving the
input impedance presented by the V
OCM
as listed in the
Electrical Characteristics Table. This impedance can be
assumed to be connected to a mid-supply potential. If an
external reference drives the V
OCM
pin, it should still be
bypassed with a high quality 0.01μF or higher capacitor to
a low impedance ground plane to fi lter any thermal noise
and to prevent common mode signals on this pin from
being inadvertently converted to differential signals.
Output Filter Considerations and Use
Filtering at the output of the LTC6403-1 is often desired
to provide either anti-aliasing or improved signal to noise
ratio. To simplify this fi ltering, the LTC6403-1 includes an
additional pair of differential outputs (+OUTF and –OUTF)
which incorporate an internal lowpass fi lter network with
a –3dB bandwidth of 44.2MHz (Figure 6).
Figure 4. Optimal Compensation for Signal Source Impedance
Figure 5. Circuit for Common Mode Range
V
S
+
–
–
+
R
F
R
F
R
I
R
INM
V
OCM
R
S
R
I
R2 = R
S
|| R1
R1 CHOSEN SO THAT R1 || R
INM
= R
S
R2 CHOSEN TO BALANCE R
S
|| R1
R1
64031 F04
V
–
V
–
V
+
0.1μF
0.1μF
0.1μF
0.1μF
0.1μF
V
CM
–
+
1
SHDN
5 6
–IN
7
+OUT
8
+OUTF
16 15
+IN
NC
NC
14
–OUT
13
–OUTF
V
–OUTF
R
F
V
+OUTF
V
–OUT
V
+OUT
2
V
+
3
V
–
V
+
V
+
V
–
V
+
V
–
4
V
OCM
V
SHDN
V
VOCM
V
OCM
12
V
–
11
V
+
10
V
+
9
V
–
V
–
V
–
64031 F05
LTC6403-1
SHDN
0.1μF
0.01μF
R
F
R
I
R
I
+
–
V
INP
–
+
V
INM
V
–IN
V
+IN
LTC6403-1
15
64031fa
These pins each have an output impedance of 100Ω. In-
ternal capacitances are 12pF to V
–
on each fi ltered output,
plus an additional 12pF capacitor connected differentially
between the two fi ltered outputs. This resistor/capacitor
combination creates fi ltered outputs that look like a se-
ries 100Ω resistor with a 36pF capacitor shunting each
fi ltered output to AC ground, providing a –3dB bandwidth
of 44.2MHz, and a noise bandwidth of 69.4MHz. The
fi lter cutoff frequency is easily modifi ed with just a few
external components. To increase the cutoff frequency,
simply add 2 equal value resistors, one between +OUT
and +OUTF and the other between –OUT and –OUTF
(Figure 7). These resistors, in parallel with the internal
100Ω resistors, lower the overall resistance and therefore
increase fi lter bandwidth. For example, to double the fi lter
bandwidth, add two external 100Ω resistors to lower the
series fi lter resistance to 50Ω. The 36pF of capacitance
remains unchanged, so fi lter bandwidth doubles. Keep in
mind, the series resistance also serves to decouple the
outputs from load capacitance. The unfi ltered outputs of
the LTC6403-1 are designed to drive 10pF to ground or
5pF differentially, so care should be taken to not lower the
effective impedance between +OUT and +OUTF or –OUT
and –OUTF below 25Ω.
To decrease fi lter bandwidth, add two external capacitors,
one from +OUTF to ground, and the other from –OUTF to
ground. A single differential capacitor connected between
APPLICATIONS INFORMATION
+OUTF and –OUTF can also be used, and since it is being
driven differentially it will appear at each fi ltered output
as a single-ended capacitance of twice the value. To halve
the fi lter bandwidth, for example, two 36pF capacitors
could be added (one from each fi ltered output to ground).
Alternatively, one 18pF capacitor could be added between
the fi ltered outputs, again halving the fi lter bandwidth.
Combinations of capacitors could be used as well; a three
capacitor solution of 12pF from each fi ltered output to
ground plus a 12pF capacitor between the fi ltered outputs
would also halve the fi lter bandwidth (Figure 8).
Figure 7. LTC6403-1 Filter Topology Modifi ed for 2x
Filter Bandwidth (2 External Resistors)
Figure 6. LTC6403-1 Internal Filter Topology Figure 8. LTC6403-1 Filter Topology Modifi ed for 1/2x
Filter Bandwidth (3 External Capacitors)
–
+
7
+OUT
8
+OUTF
14
–OUT
13
–OUTF
12
V
–
9
V
–
V
–
V
–
64031 F06
LTC6403-1
FILTERED OUTPUT
(44.2MHz)
100Ω
12pF
12pF
12pF
100Ω
–
+
7
+OUT
8
+OUTF
14
–OUT
13
–OUTF
12
V
–
9
V
–
V
–
V
–
64031 F07
LTC6403-1
FILTERED OUTPUT
(88.4MHz)
100Ω
100Ω
12pF
12pF
12pF
100Ω
100Ω
–
+
7
+OUT
8
+OUTF
14
–OUT
13
–OUTF
12
V
–
9
V
–
V
–
V
–
64031 F08
LTC6403-1
FILTERED OUTPUT
(22.1MHz)
100Ω
12pF
12pF
12pF
12pF
12pF
100Ω
12pF
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LTC6403HUD-1#PBF

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Buy LTC6403HUD-1#PBF
Manufacturer:
Analog Devices Inc.
Description:
Differential Amplifiers 200MHz Low Noise ADC Driver
Lifecycle:
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