ammp-6650 Avago Technologies, ammp-6650 Datasheet - Page 8

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

Biasing­considerations
RFin
Note : Package base : GND
Figure­13.­Bias­voltage­connections
Attenuation is controlled by applying voltage to pin V1
(Pin 7) and pin V2 (Pin5), as shown in Figure 13.
For the minimum attenuation, V1 is set to 1.5 V and V2
is set to 0 V. The 1.5 V applied to the V1 pin biases the
series FETs to a full “on” state, while the 0 V applied to the
V2 pin keeps the shunt FETs in an “off” or “open” state;
thus creating the lumped element 50 Ω transmission line
effect. The V2 voltage swing from 0 V to 1.25 V increases
the level of attenuation. The V1 voltage swing from 1.5 V
to 0 V effectively optimizes the input and output match at
higher attenuation levels. The AMMP-6650 can be driven
by two complementary voltage ramps placed on V1 and
V2. Careful adjustments of the two control lines over a
relatively small voltage ranges are required to set the at-
tenuation and optimize VSWR.
The on-chip DC reference circuit can be used to optimize
VSWR for any attenuation setting, improve voltage versus
attenuation linearity and range, and provide temperature
compensation.
The on-chip DC reference circuit is a non-distributed “T”
attenuator designed to operate in a 500 Ω system and
track the control voltage versus attenuation character-
istics of the RF attenuator. A simplified schematic of the
AMMP-6650 together with an op-amp driver that utilizes
the DC reference circuit is shown in Figure 14.
OP­ AMP­ 1 insures that the attenuator maintains a good
input and output match to 50 Ω, while OP­AMP­2 increases
the usable control voltage range versus using only direct
voltage ramps for V1 and V2 and improves over tempera-
ture operation.
8
8
DCin
V1
1
7
reference circuit
attenuator
Top View
variable
NC
NC
DC
2
6
DCout
V2
5
3
4
RFout
Pin
Pin
1
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
Function
Function
DC out
DC out
RF out
RF out
DC in
DC in
RF in
RF in
V2
V2
V1
V1
R
Figure­14.­AMMP-6650­and­the­op-amp­driver­circuit
If optimum VSWR is all that is required, OP­AMP­2 may be
eliminated however, R
DCout pad of the AMMP-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
circuit DCin pad via a 500 Ω resistor, creating a 500 Ω
source. The reference circuit termination R
to the DCout pad and ideally is also equal to 500 Ω. This
voltage is controlled in parallel with the RF attenuator.
The chosen value of V
modifying the FET biasing and lower than the turn-on
voltage of the ESD protection diode but high enough such
that the attenuated voltage at OP­AMP­2 is usable compared
to input offsets 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
When V2 is changed to a lower value, the voltage at node
A becomes greater than that of node B. This voltage dif-
ference 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 OP­AMP­1 will swing toward its
negative rail until equilibrium 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 50 Ω.
V
REF
REF
(620)
500
R
500
S
A
B
RFin
OP AMP 1
+
_
C1
DCin
V1
REF
L
NC
NC
must remain connected to the
must be low enough to avoid
V2
REF
DCout
) is fed to the reference
OP AMP 2
RFout
+
_
C
D
L
, is connected
R
R1 (10K)
R2 (100)
L
V
(500)
CONTROL
REF
.