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LT6604IUFF-5#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LT6604-5
10
66045fa
APPLICATIONS INFORMATION
Use Figure 4 to determine the interface between the
LT6604-5 and a current output DAC. The gain, or “tran-
simpedance,” is defi ned as A = V
OUT
/I
IN
. To compute the
transimpedance, use the following equation:
A =
806 •R1
R1+ R2
Ω
By setting R1 + R2 = 806Ω, the gain equation reduces to A
= R1(Ω). The voltage at the pins of the DAC is determined
by R1, R2, the voltage on V
MID
and the DAC output current.
Consider Figure 4 with R1 = 49.9Ω and R2 = 750Ω. The
voltage at V
MID
, for V
S
= 3.3V, is 1.65V. The voltage at the
DAC pins is given by:
V
DAC
= V
MID
•
R1
R1+ R2 + 806
+ I
IN
R1• R2
R1+ R2
= 51mV + I
IN
46.8Ω
Evaluating the LT6604-5
The low impedance levels and high frequency operation
of the LT6604-5 require some attention to the matching
networks between the LT6604-5 and other devices. The
previous examples assume an ideal (0Ω) source impedance
and a large (1k) load resistance. Among practical examples
where impedance must be considered is the evaluation of
the LT6604-5 with a network analyzer.
Figure 5 is a laboratory setup that can be used to character-
ize the LT6604-5 using single-ended instruments with 50Ω
source impedance and 50Ω input impedance. For a unity
gain confi guration the LT6604-5 requires an 806Ω source
resistance yet the network analyzer output is calibrated
for a 50Ω load resistance. The 1:1 transformer, 51.1Ω
and 787Ω resistors satisfy the two constraints above.
The transformer converts the single-ended source into a
differential stimulus. Similarly, the output of the LT6604-5
will have lower distortion with larger load resistance yet
the analyzer input is typically 50Ω. The 4:1 turns (16:1
impedance) transformer and the two 402Ω resistors of
Figure 5, present the output of the LT6604-5 with a 1600Ω
differential load, or the equivalent of 800Ω to ground at
each output. The impedance seen by the network analyzer
input is still 50Ω, reducing refl ections in the cabling be-
tween the transformer and analyzer input.
Differential and Common Mode Voltage Ranges
The differential amplifi ers inside the LT6604-5 contain
circuitry to limit the maximum peak-to-peak differential
voltage through the fi lter. This limiting function prevents
excessive power dissipation in the internal circuitry and
provides output short-circuit protection. The limiting
function begins to take effect at output signal levels
above 2V
P-P
and it becomes noticeable above 3.5V
P-P
.
This is illustrated in Figure 6; the LT6604-5 channel was
confi gured with unity passband gain and the input of the
fi lter was driven with a 1MHz signal. Because this voltage
limiting takes place well before the output stage of the
fi lter reaches the supply rails, the input/output behavior
of the IC shown in Figure 6 is relatively independent of
the power supply voltage.
–
+
0.1μF
3.3V
–
+
0.01μF
CURRENT
OUTPUT
DAC
V
OUT
+
V
OUT
–
66045 F04
R2
R1
I
IN
–
I
IN
+
R2
R1
=
V
OUT
+
– V
OUT
–
I
IN
+
– I
IN
–
806 • R1
R1 + R2
25
27
4
34
6
2
29
7
1/2
LT6604-5
Figure 4
–
+
0.1μF
0.1μF
2.5V
–2.5V
–
+
66045 F05
402Ω
402Ω
NETWORK
ANALYZER
INPUT
50Ω
COILCRAFT
TTWB-16A
4:1
NETWORK
ANALYZER
SOURCE
COILCRAFT
TTWB-1010
1:1
50Ω
51.1Ω
787Ω
787Ω
25
27
4
34
6
2
29
7
1/2
LT6604-5
Figure 5
LT6604-5
11
66045fa
APPLICATIONS INFORMATION
The two amplifi ers inside the LT6604-5 channel have in-
dependent control of their output common mode voltage
(see the Block Diagram section). The following guidelines
will optimize the performance of the fi lter.
V
MID
can be allowed to fl oat, but it must be bypassed to an
AC ground with a 0.01μF capacitor or some instability may
be observed. V
MID
can be driven from a low impedance
source, provided it remains at least 1.5V above V
–
and at
least 1.5V below V
+
. An internal resistor divider sets the
voltage of V
MID
. While the internal 11k resistors are well
matched, their absolute value can vary by ±20%. This
should be taken into consideration when connecting an
external resistor network to alter the voltage of V
MID
.
V
OCM
can be shorted to V
MID
for simplicity. If a different
common mode output voltage is required, connect V
OCM
to a voltage source or resistor network. For 3V and 3.3V
supplies the voltage at V
OCM
must be less than or equal
to the mid supply level. For example, voltage (V
OCM
) ≤
1.65V on a single 3.3V supply. For power supply voltages
higher than 3.3V the voltage at V
OCM
can be set above mid
supply. The voltage on V
OCM
should not be more than 1V
below the voltage on V
MID
. The voltage on V
OCM
should
not be more than 2V above the voltage on V
MID
. V
OCM
is
a high impedance input.
The LT6604-5 was designed to process a variety of input
signals including signals centered on the mid supply
voltage and signals that swing between ground and a
positive voltage in a single supply system (Figure 1). The
allowable range of the input common mode voltage (the
average of V
IN
+
and V
IN
–
in Figure 1) is determined by
the power supply level and gain setting (see the Electrical
Characteristics section).
Common Mode DC Currents
In applications like Figure 1 and Figure 3 where the LT6604-5
not only provides lowpass fi ltering but also level shifts the
common mode voltage of the input signal, DC currents
will be generated through the DC path between input and
output terminals. Minimize these currents to decrease
power dissipation and distortion.
Consider the application in Figure 3. V
MID
sets the output
common mode voltage of the 1st differential amplifi er
inside the LT6604-5 (see the Block Diagram section) at
2.5V. Since the input common mode voltage is near 0V,
there will be approximately a total of 2.5V drop across the
series combination of the internal 806Ω feedback resistor
and the external 200Ω input resistor. The resulting 2.5mA
common mode DC current in each input path, must be
absorbed by the sources V
IN
+
and V
IN
–
. V
OCM
sets the
common mode output voltage of the 2nd differential
amplifi er inside the LT6604-5 channel, and therefore sets
the common mode output voltage of the fi lter. Since, in
the example of Figure 3, V
OCM
differs from V
MID
by 0.5V,
an additional 1.25mA (625μA per side) of DC current will
fl ow in the resistors coupling the 1st differential amplifi er
output stage to the fi lter output. Thus, a total of 6.25mA
is used to translate the common mode voltages.
A simple modifi cation to Figure 3 will reduce the DC com-
mon mode currents by 36%. If V
MID
is shorted to V
OCM
the
common mode output voltage of both op amp stages will
be 2V and the resulting DC current will be 4mA. Of course,
by AC coupling the inputs of Figure 3 and shorting V
MID
to
V
OCM
, the common mode DC current is eliminated.
Figure 6. Differential Voltage Range
1MHz INPUT LEVEL (V
P-P
)
0
20
0
–20
–40
–60
–80
–100
–120
35
6600 F06
12
467
OUTPUT LEVEL (dBV)
1dB PASSBAND GAIN
COMPRESSION POINTS
1MHz T
A
= 25°C
1MHz T
A
= 85°C
3RD HARMONIC
T
A
= 85°C
3RD HARMONIC
T
A
= 25°C
2ND HARMONIC
T
A
= 25°C, GAIN = 1
2ND HARMONIC
T
A
= 85°C
LT6604-5
12
66045fa
APPLICATIONS INFORMATION
Noise
The noise performance of the LT6604-5 channel can be
evaluated with the circuit of Figure 7. Given the low noise
output of the LT6604-5 and the 6dB attenuation of the
transformer coupling network, it is necessary to measure
the noise fl oor of the spectrum analyzer and subtract the
instrument noise from the fi lter noise measurement.
Example: With the IC removed and the 25Ω resistors
grounded, Figure 7, measure the total integrated noise
(e
S
) of the spectrum analyzer from 10 kHz to 5MHz. With
the IC inserted, the signal source (V
IN
) disconnected, and
the input resistors grounded, measure the total integrated
noise out of the fi lter (e
O
). With the signal source connected,
set the frequency to 1MHz and adjust the amplitude until
V
IN
measures 100mV
P-P
. Measure the output amplitude,
V
OUT
, and compute the passband gain A = V
OUT
/V
IN
. Now
compute the input referred integrated noise (e
IN
) as:
e
IN
=
(e
O
)
2
–(e
S
)
2
A
Table 1 lists the typical input referred integrated noise for
various values of R
IN
.
Table 1. Noise Performance
PASSBAND
GAIN R
IN
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 5MHz
INPUT REFERRED
NOISE dBm/Hz
4 200Ω 24μV
RMS
–149
2 402Ω 38μV
RMS
–145
1 806Ω 69μV
RMS
–140
Figure 8 is plot of the noise spectral density as a function
of frequency for an LT6604-5 with R
IN
= 806Ω using the
fi xture of Figure 7 (the instrument noise has been sub-
tracted from the results).
The noise at each output is comprised of a differential
component and a common mode component. Using a
transformer or combiner to convert the differential outputs
to single-ended signal rejects the common mode noise and
gives a true measure of the S/N achievable in the system.
Conversely, if each output is measured individually and the
noise power added together, the resulting calculated noise
level will be higher than the true differential noise.
Power Dissipation
The LT6604-5 amplifi ers combine high speed with large
signal currents in a small package. There is a need to en-
sure that the die’s junction temperature does not exceed
150°C. The LT6604-5 has an exposed pad (pin 35) which
is connected to the lower supply (V
–
). Connecting the pad
to a ground plane helps to dissipate the heat generated
by the chip. Metal trace and plated through-holes can be
used to spread the heat generated by the device to the
backside of the PC board.
–
+
0.1μF
0.1μF
2.5V
–2.5V
–
+
R
IN
R
IN
25Ω
25Ω
66045 F07
SPECTRUM
ANALYZER
INPUT
50Ω
V
IN
COILCRAFT
TTWB-1010
1:1
25
27
4
34
6
2
29
7
1/2
LT6604-5
Figure 7
Figure 8. Input Referred Noise
FREQUENCY (MHz)
0.01
NOISE DENSITY (nV/√Hz)
INTEGRATED NOISE (μV)
100
45
40
35
30
25
20
15
10
5
0
90
80
70
60
50
40
30
20
10
0
66045 F08
0.1 101
INTEGRATED NOISE, GAIN = 1X
INTEGRATED NOISE, GAIN = 4X
NOISE DENSITY, GAIN = 1X
NOISE DENSITY, GAIN = 4X
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LT6604IUFF-5#TRPBF

Mfr. #:
Buy LT6604IUFF-5#TRPBF
Manufacturer:
Analog Devices Inc.
Description:
Differential Amplifiers Dual Differential Amplifier and 5MHz Lowpass Filter
Lifecycle:
New from this manufacturer.
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