LT6604-15
11
660415fb
APPLICATIONS INFORMATION
where impedance must be considered is the evaluation of
the LT6604-15 with a network analyzer.
Figure 5 is a laboratory setup that can be used to char-
acterize the LT6604-15 using single-ended instruments
with 50 source impedance and 50 input impedance.
For a unity gain confi guration the LT6604-15 requires
an 536 source resistance yet the network analyzer
output is calibrated for a 50 load resistance. The 1:1
transformer, 52.3 and 523 resistors satisfy the two
constraints above. The transformer converts the single-
ended source into a differential stimulus. Similarly, the
output of the LT6604-15 will have lower distortion with
larger load resistance yet the analyzer input is typically
50. The 4:1 turns (16:1 impedance) transformer and the
two 402 resistors of Figure 5, present the output of the
LT6604-15 with a 1600 differential load, or the equiva-
lent of 800 to ground at each output. The impedance
seen by the network analyzer input is still 50, reducing
refl ections in the cabling between the transformer and
analyzer input.
Differential and Common Mode Voltage Ranges
The differential amplifi ers inside the LT6604-15 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 LT6604-15 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.
The two amplifi ers inside the LT6604-15 channel have
independent 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 LT6604-15 was designed to process a variety of input
signals including signals centered on the mid-supply volt-
age and signals that swing between ground and a positive
voltage in a single supply system (Figure 1). The range of
allowable 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 Distortion vs Input Common
Mode Level in the Typical Performance Characteristics).
1MHz INPUT LEVEL (V
P-P
)
0
20
0
–20
–40
–60
–80
–100
35
660415 F06
12
467
OUTPUT LEVEL (dBV)
3RD HARMONIC
85°C
1dB COMPRESSION
POINTS
25°C
85°C
3RD HARMONIC
25°C
2ND HARMONIC, 25°C
2ND
HARMONIC
85°C
Figure 6. Output Level vs Input Level, Differential
1MHz Input, Gain = 1
