Rainbow Electronics MAX2063 - Manual (Page 18)

MAX2063
Dual 50MHz to 1000MHz High-Linearity,
Serial/Parallel-Controlled Digital VGA
18
Table 3. SPI Data Format (continued)
Digital Attenuator Settings Using
the Parallel Control Bus
To capitalize on its fast 25ns switching capability, the
device offers a supplemental 5-bit parallel control inter-
face for each attenuator. The two buses of the digital
logic attenuator-control pins (D0_ _–D4_ _) enable the
attenuator stages (Table 4).
Direct access to these 5-bit buses enables the user to
avoid any programming delays associated with the SPI
interface. One of the limitations of any SPI bus is the
speed at which commands can be clocked into each
peripheral device. By offering direct access to the 5-bit
parallel interface, the user can quickly shift between
digital attenuator states as needed for critical “fast-
attack” automatic gain-control (AGC) applications.
Note that when the digital attenuators are controlled
by the SPI bus, the control voltages of each digital
attenuator show on the five parallel control pins (pins
14–17, 19 for digital attenuator 2, and pins 42, 44–47 for
digital attenuator 1). When the digital attenuators are in
SPI mode, the parallel control pins must be open.
“Rapid-Fire” Preprogrammed
Attenuation States
The device has an added feature that provides
“rapid-fire” gain selection between four preprogrammed
attenuation steps. As with the supplemental 5-bit buses
previously mentioned, this “rapid-fire” gain selection
allows the user to quickly access any one of four
customized digital attenuation states without incurring
the delays associated with reprogramming the device
through the SPI bus.
The switching speed is comparable to that achieved
using the supplemental 5-bit parallel buses. However,
by employing this specific feature, the digital attenuator
I/O is further reduced by a factor of either 5 or 2.5 (5
control bits vs. 1 or 2, respectively), depending on the
number of states desired.
The user can employ the STA_A_1 and STA_B_1
(STA_A_2 and STA_B_2 for attenuator 2) logic input pins
to apply each step as required (see Tables 5 and 6).
Toggling just the STA_A_1 pin (1 control bit) yields two
preprogrammed attenuation states; toggling both the
STA_A_1 and STA_B_1 pins together (2 control bits)
yields four preprogrammed attenuation states.
As an example, assume that the AGC application
requires a static attenuation adjustment to trim out
gain inconsistencies within a receiver lineup. The same
AGC circuit can also be called upon to dynamically
attenuate an unwanted blocker signal that could desense
the receiver and lead to an ADC overdrive condition. In
this example, the device would be preprogrammed
(through the SPI bus) with two customized attenuation
states—one to address the static gain-trim adjustment,
the second to counter the unwanted blocker condition.
Toggling just the STA_A_1 control bit enables the
user to switch quickly between the static and dynamic
attenuation settings with only one I/O pin.
FUNCTION BIT DESCRIPTION
1st Digital Attenuator
State 1
D12 16dB step (MSB of the 5-bit word used to program the digital attenuator state 1)
D11 8dB step
D10 4dB step
D9 2dB step
D8 1dB step
Reserved
D7
Bits D[7:0] are reserved. Set to logic 0.
D6
D5
D4
D3
D2
D1
D0 (LSB)