9
OP AMP 1 insures that the attenuator maintains a good
input and output match to 50Ω, while OP AMP 2 increases
the usable control voltage range versus using only direct
voltage ramps for V1 and V2 and improves over tempera-
ture operation.
If optimum VSWR is all that is required, OP AMP 2 may be
eliminated however, R
L
must remain connected to the
DCout
pad of the AMMC-6650 and the control voltage can be
applied directly to V2.
CAUTION: Low voltage op-amps must be used so as not to
exceed the maximum limit of V1 and V2 control voltages.
As shown, a voltage reference (V
REF
) is fed to the reference
circuit DCin pad via a 500Ω resistor, creating a 500Ω source.
The reference circuit termination R
L
, is connected to the
DCout pad and ideally is also equal to 500Ω. This voltage
is controlled in parallel with the RF attenuator. The chosen
value of V
REF
must be low enough to avoid modifying the
FET biasing and lower than the turn-on voltage of the ESD
protection diode but high enough such that the attenuat-
ed voltage at OP AMP 2 is usable compared to input o sets
etc. The optimum value for the positive reference voltage
is approximately 0.1 to 0.4 V.
At equilibrium, the voltages at nodes A and B of the OP AMP 1
must be equal which implies that the input impedance
to the DC reference circuit is equal to R
REF
. When V2 is
changed to a lower value, the voltage at node A becomes
greater than that of node B. This voltage di erence causes
the output voltage of op OP AMP 1 to move toward its
positive rail until equilibrium is once again established.
When V2 is changed to a higher value the voltage at node
A becomes less than that of node B and the output voltage
of OP AMP 1 will swing toward its negative rail until equi-
librium is reached. If the reference circuit precisely tracks
the RF circuit, the voltage output of OP AMP 1 at equilibrium
insures that the RF circuit is matched to 50Ω.
If attenuation linearity is required, OP AMP 2 is included
as shown in Figure 14 and a positive control voltage is
applied to V
CONTROL
.
At equilibrium, voltages at nodes C and D are equal.
When V
CONTROL
is changed, the output 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
CONTROL
. The input
voltage to the reference circuit is being held constant and
the log(V
CONTROL
) is proportional 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
CONTROL
).
Two RF attenuation vs voltage curves corresponding
to di erent values of V
REF
are shown in Figure 15. These
curves were obtained by using the driver circuit shown in
Figure 14 and the V
REF
values 0.1 V and 0.4 V.
Values for R
L
, R1 and R2 were 500Ω, 10 kΩ and 100Ω re-
spectively. 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.
OP AMP 2 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 OP AMP 2.
Another way to improve performance of the attenuator
driver circuit is to adjust R
L
and R
REF
. If the reference circuit
precisely tracked the RF circuit and the ON resistance
of the FETs was zero ohms, then R
L
and R
REF
would be
exactly 500Ω. Due to the di erence in layout structures,
the reference circuit does not track the RF circuit precisely.
R
L
and R
REF
can be adjusted in order to compensate for
these di erences. Optimum values of R
L
and R
REF
have
been found to be between 500Ω and 650Ω.
For maximum dynamic range on the attenuation control
circuit, R
L
should be less than R
REF
by an amount equal
to the “ON resistance” of the reference circuit series FETs.
The “ON resistance” of the series FETs is about 95Ω total.
Therefore, the relationship between R
L
and R
REF
is as
follows:
R
REF
= R
L
+ 95Ω
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
CONTROL
equal
to 0 V. Minimum attenuation is achieved when
V
control
≈ x x V
ref
or
V
control
≈ x DC
out
Therefore, an increase in the resistor ratio R1/R2 increases
the value of the control voltage required to produce
minimum attenuation.
R1 + R2
R2
⎛
⎜
⎝
⎛
⎜
⎝
R
L
500 Ω + R
L
⎛
⎜
⎝
⎛
⎜
⎝
R1
R2
⎛
⎜
⎝
⎛
⎜
⎝
1 +
