ammp-6650 Avago Technologies, ammp-6650 Datasheet - Page 9

• Manufacturer Part Number: ammp-6650
• Description: Dc ? 30 Ghz Variable Attenuator
• Manufacturer: Avago Technologies

If attenuation linearity is required, OP­ AMP­ 2 is included
as shown in Figure 14 and a positive control voltage is
applied to V
and D are equal. When V
of OP­ AMP­ 2 adjusts to a value that forces the voltage at
node C to equal the voltage at node D. Therefore, the
output voltage of the DC reference circuit is proportional
to V
being held constant and the log(V
to the reference circuit attenuation 20log(DCout/DCin).
If the FET parameters of the DC reference circuit track the
FET parameters of the RF circuit, the voltage output of the
RF circuit is also proportional to the control voltage. This
translates to a linear relationship between the attenuation
(in dB) and the log(V
Two RF attenuation vs voltage curves corresponding
to different values of V
curves were obtained by using the driver circuit shown in
Figure 14 and the V
R
Control voltage ranged from 4.5 V to 0 V.
Because the FETs in the DC circuit are not identical to
those in the RF circuit, the DC circuit does not exactly track
the RF circuit. This results in attenuation vs. voltage curves
that are not exactly linear.
OP­AMP­2 provides temperature compensation by adjusting
V2 in such a way as to keep voltage at point C equal to
that point D. If the attenuation changes over tempera-
ture, voltage at point C tries to change, but is corrected by
OP­AMP­2.
Another way to improve performance of the attenua-
tor driver circuit is to adjust R
circuit precisely tracked the RF circuit and the ON resis-
tance of the FETs was zero ohms, then R
be exactly 500 Ω.
Due to the difference in layout structures, the reference
circuit does not track the RF circuit precisely. R
can be adjusted in order to compensate for these differ-
ences. Optimum values of R
to be between 500 Ω and 650 Ω.
For maximum dynamic range on the attenuation control
circuit, R
to the “ON resistance” of the reference circuit series FETs.
The “ON resistance” of the series FETs is about 95 Ω total.
Therefore, the relationship between R
follows:
9
L
, R1 and R2 were 500 Ω, 10 kΩ and 100 Ω respectively.
CONTROL
R
L
REF
should be less than R
. The input voltage to the reference circuit is
CONTROL
= R
L
+ 95 Ω
. At equilibrium, voltages at nodes C
REF
CONTROL
REF
values 0.1 V and 0.4 V. Values for
CONTROL
are shown in Figure 15. These
).
L
L
and R
and R
REF
CONTROL
is changed, the output
REF
by an amount equal
REF
have been found
L
L
. If the reference
) is proportional
and R
and R
L
REF
REF
and R
would
is as
REF
The voltage divider formed by R1 and R2 can be used
to adjust the sensitivity of the attenuator versus control
voltage. For the driver circuit shown in Figure 14, maximum
attenuation is always achieved by setting V
to 0 V. Minimum attenuation is achieved when
V
or
V
Therefore, an increase in the resistor ratio R1/R2 increases
the value of the control voltage required to produce
minimum attenuation.
LMV932 (National Semiconductor) was used in the control
circuit that produced the results shown in Figure 15;
however, any low noise, low offset voltage op amp should
produce similar results. LMV932’s low supply voltage of
1.8 volts, limits the possibility of exceeding the 1.5 volt
absolute maximum of the AMMC-6650 V1 and V2 control
line inputs.
Figure­15.­Attenuation­vs.­Control­Voltage­@­15­GHz
Recommended­SMT­Attachment
The AMMP Packaged Devices are compatible with high
volume surface mount PCB assembly processes.
The PCB material and mounting pattern, as defined in
Figure 19, optimizes RF performance and is strongly
recommended. An electronic drawing of the land pattern
is available upon request from Avago Sales & Application
Engineering.
control
control
-10
-15
-20
-25
-30
-35
-40
-45
-50
-5
0
0.01
≈
≈






1 +
R1 + R2
R2
Vref=0.1V
Vref=0.2V
R1
R2






x
x DC
0.1



Control Voltage (Vin)
500 Ω + R
out
R
L
L



x V
ref
1
CONTROL
equal
10