AD8341-EVAL Analog Devices Inc, AD8341-EVAL Datasheet - Page 13

• Manufacturer Part Number: AD8341-EVAL
• Manufacturer: Analog Devices Inc

Specifications of AD8341-EVAL

• Lead Free Status / Rohs Status: Not Compliant

RF OUTPUT AND MATCHING
The RF outputs of the AD8341, RFOP, and RFOM, are open
collectors of a transimpedance amplifier which need to be
pulled up to the positive supply, preferably with RF chokes as
shown in Figure 31. The nominal output impedance looking
into each individual output pin is 25 Ω. Consequently, the
differential output impedance is 50 Ω.
Since the output dc levels are at the positive supply, ac coupling
capacitors will usually be needed between the AD8341 outputs
and the next stage in the system.
A 1:1 RF broadband output balun, such as the ETC1-1-13
(M/A-COM), converts the differential output of the AD8341
into a single-ended signal. Note that the loss and balance of the
balun directly impact the apparent output power, noise floor,
and gain/phase errors of the AD8341. In critical applications,
narrow-band baluns with low loss and superior balance are
recommended.
If the output is taken in a single-ended fashion directly into a
50 Ω load through a coupling capacitor, there will be an imped-
ance mismatch. This can be resolved with a 1:2 balun to convert
the single-ended 25 Ω output impedance to 50 Ω. If loss of
signal swing is not critical, a 25 Ω back termination in series
with the output pin can also be used. The unused output pin
must still be pulled up to the positive supply. The user may load
it through a coupling capacitor with a dummy load to preserve
balance. The gain of the AD8341 when the output is single-
ended varies slightly with dummy load value as shown in Figure 32.
±I
SIG
Figure 31. RF Output Interface to the AD8341 Showing
Coupling Capacitors, Pull-Up RF Chokes, and Balun
R
R
G
T
T
M
RFOM
RFOP
DIFFERENTIAL
120nH
V
50Ω
P
100pF
100pF
1:1
RF
OUTPUT
Rev. 0 | Page 13 of 20
The RF output signal can be disabled by raising the DSOP pin
to the positive supply. The output disable function provides
>30 dB attenuation of the input signal even at full gain. The
interface to DSOP is high impedance and the shutdown and
turn-on response times are <100 ns. If the disable function is
not needed, the DSOP pin should be tied to ground.
DRIVING THE I-Q BASEBAND CONTROLS
The I and Q inputs to the AD8341 set the gain and phase be-
tween input and output. These inputs are differential and should
normally have a common-mode level of 0.5 V. However, when
differentially driven, the common mode can vary from 250 mV
to 750 mV while still allowing full gain control. Each input pair
has a nominal input swing of ±0.5 V differential around the
common-mode level. The maximum gain of unity is achieved if
the differential voltage is equal to +500 mV or −500 mV. So
with a common-mode level of 500 mV, IBBP and IBBM will
each swing between 250 mV and 750 mV.
The I and Q inputs can also be driven with a single-ended
signal. In this case, one side of each input should be tied to a
low noise 0.5 V voltage source (a 0.1 µF decoupling capacitor
located close to the pin is recommended), while the other input
swings from 0 V to 1 V. Differential drive generally offers superior
even-order distortion and lower noise than single-ended drive.
The bandwidth of the baseband controls exceeds 200 MHz even
at full-scale baseband drive. This allows for very fast gain and
phase modulation of the RF input signal. In cases where lower
modulation bandwidths are acceptable or desired, external filter
capacitors can be connected across Pins IFLP to IFLM and
QFLP to QFLM to reduce the ingress of baseband noise and
spurious signal into the control path.
Figure 32. Gain of the AD8341 Using a Single-Ended Output with Different
–2.5
–3.0
–3.5
–4.0
–4.5
–5.0
–5.5
–6.0
–6.5
–7.0
–7.5
–8.0
–8.5
1.0
1.2
Dummy Loads, R
1.4
1.6
FREQUENCY (GHz)
R
R
L2
1.8
L2
L2
, on the Unused Output
= SHORT
R
= OPEN
L2
2.0
= 50 Ω
2.2
2.4
2.6
R
L
AD8341
2.8
= 50 Ω
3.0