AD6636(Rev0) Просмотр технического описания (PDF) - Analog Devices
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AD6636 Datasheet PDF : 72 Pages
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AD6636
The PLL clock multiplier is programmable and uses input clock
rates between 4 MHz and 150 MHz to give a system clock rate
(output) of as high as 200 MHz.
The output clock rate is given by
PLL _ CLK = CLKA× M
N
where:
CLKA is the Input Port A clock rate.
M is a 5-bit programmable multiplication factor.
N is a predivide factor.
M is a 5-bit number between 4 and 20 (both values included). N
(predivide) can be 1, 2, 4, or 8. The multiplication factor M is
programmed using a 5-bit PLL clock multiplier word in the
ADC clock control register. A value outside the valid range of 4
to 20 bypasses the PLL clock multiplier and, therefore, the PLL
clock is the same as the input clock. The predivide factor N is
programmed using a 2-bit ADC pre-PLL clock divider word in
the ADC clock control register, as listed in Table 10.
Table 10. PLL Clock Generation Predivider Control
Predivide Word [1:0]
Divide-by Value for the Clock
00
Divide-by-1, bypass
01
Divide-by-2
10
Divide-by-4
11
Divide-by-8
For best signal processing advantage, the user should program
the clock multiplier to give a system clock output as close as
possible to, but not exceeding, 200 MHz. The internal blocks of
the AD6636 that run off of the PLL clock are rated to run at a
maximum of 200 MHz. The default power-up state for the PLL
clock multiplier is the bypass state, where CLKA is passed on as
the PLL clock.
ADC GAIN CONTROL
Each ADC input port has individual, high speed gain-control
logic circuitry. Such gain-control circuitry is useful in applica-
tions that involve large dynamic-range inputs or in which
gain-ranging ADCs are employed. The AD6636 gain-control
logic allows programmable upper and lower thresholds and a
programmable dwell-time counter for temporal hysteresis.
Each input port has a 3-bit output from the gain control block.
These three output pins are shared with the 3-bit exponent
input pins for each input port. The operation is controlled by
the gain control enable bit in gain control register of the
individual input ports. A Logic 1 in this bit programs the
EXP[2:0] pins as gain-control outputs, and a Logic 0 configures
the pins as input exponent pins. To avoid bus contention, these
pins are set, by default, as input exponent pins.
Function
The gain-control block features a programmable upper
threshold register and a lower threshold register. The ADC
input data is compared to both these registers. If ADC input
data is larger than the upper threshold register, then the gain
control output is decremented by 1. If ADC input data is smaller
than the lower threshold register, then the gain control output is
incremented by 1. When decrementing the gain control output,
the change is immediate. But when incrementing the output, a
dwell-time register is used to delay the change. If the ADC input
is larger than the upper threshold register value, the gain-
control output is decremented immediately to prevent overflow.
When the ADC input is lower than the lower threshold register,
a dwell timer is loaded with the value in the programmable
20-bit dwell-time register. The counter decrements once every
input clock cycle, as long as the input signal remains below the
lower threshold register value. If the counter reaches 1, the gain
control output is incremented by 1. If the signal goes above the
lower threshold register value, the gain adjustment is not made,
and the normal comparison to lower and upper threshold
registers is initiated once again. Therefore, the dwell timer
provides temporal hysteresis and prevents the gain from
switching continuously.
In a typical application, if the ADC signal goes below the lower
threshold for a time greater than the dwell time, then the gain
control output is incremented by 1. Gain control bits control the
gain ranging block, which appears before the ADC in the signal
chain. With each increment of the gain control output, gain in
the gain-ranging block is increased by 6.02 dB. This increases
the dynamic range of the input signal into the ADC by 6.02 dB.
This gain is compensated for in the AD6636 by relinearizing, as
explained in the Relinearization section. Therefore, the AD6636
can increase the dynamic range of the ADC by 42 dB, provided
that the gain-ranging block can support it.
Relinearization
The gain in the gain-ranging block (external) is compensated
for by relinearizing, using the exponent bits EXP[2:0] of the
input port. For this purpose, the gain control bits are connected
to the EXP[2:0] bits, providing an attenuation of 6.02 dB for
every increase in the gain control output. After the gain in the
external gain-ranging block and the attenuation in the AD6636
(using EXP bits), the signal gain is essentially unchanged. The
only change is the increase in the dynamic range of the ADC.
External gain-ranging blocks or gain-ranging ADCs have a
delay associated with changing the gain of the signal. Typically,
these delays can be up to 14 clock cycles. The gain change in the
AD6636 (via EXP[2:0]) must be synchronized with the gain
change in the gain-ranging block (external). This is allowed in
the AD6636 by providing a flexible delay, programmable 6-bit
word in the gain control register. The value in this 6-bit word
gives the delay in input clock cycles. A programmable pipeline
Rev. 0 | Page 21 of 72
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