AD6600ST/PCB Analog Devices Inc, AD6600ST/PCB Datasheet - Page 15

• Manufacturer Part Number: AD6600ST/PCB
• Description: BOARD EVAL ADC RSSI W/AD6600AST
• Manufacturer: Analog Devices Inc

Specifications of AD6600ST/PCB

• Rohs Status: RoHS non-compliant

THEORY OF OPERATION
The AD6600, dual-channel, gain-ranging ADC integrates ana-
log IF circuitry with high speed data conversion. Each analog
input stage is a 1 GHz, 0 dB to –24 dB, phase-compensated step
attenuator; the step size in each attenuator is 12 dB. Both input
stages drive an analog multiplex function followed by a 12 dB/
18 dB gain amplifier. A simple LC noise filter at the output of
the gain amplifier is required to resonate at the desired IF. This
resonant filter port precedes a wide input bandwidth (450 MHz)
track-and-hold followed by an 11-bit analog-to-digital converter
(ADC). A high speed synchronous peak detector monitors sig-
nal strength at both input channels. The peak detector drives
RSSI circuitry that automatically adjusts attenuation and gain
on a clock-by-clock basis. The three RSSI indicator bits and the
eleven ADC bits are available at the output providing an exponent
and mantissa data format. Together these integrated components
form an IF sampling, high dynamic range ADC system.
It is helpful to view this device as a stand-alone ADC using
automatic gain control. The gain control referred to in this data
sheet as “gain-ranging” works to maintain a constant SNR over
as wide a range as possible.
As stated previously, the AD6600 has a floating-point output:
eleven mantissa bits and three exponent bits. As shown in Fig-
ure 16, at the lowest input levels SNR increases 1 dB for a 1 dB
increase in input power. In this range, the AD6600 is set for
maximum gain. However, when the input signal level reaches
the gain-ranging section (approximately –42 dBFS), the SNR is
contained between about 50 dB and 56 dB or between 44 dB and
56 dB including the effects of hysteresis. Although Figure 16
does not indicate so, there are slight differences between the
SNR from one gain range to the next as the gain amp switches
between 12 dB and 18 dB. Once the final RSSI range has been
exceeded (approximately –12 dBFS), SNR again increases 1 dB
per 1 dB input power increase until converter full scale is reached.
Again, this performance is very much like the effects of a typical
analog AGC loop.
AIN
AIN
BIN
BIN
DETECT
PEAK
0dB, –12dB, –24dB
0dB, –12dB, –24dB
A_SEL
RSSI
ATTEN
SET
ATTEN
B_SEL
GAIN
GAIN
3
RSSI
SELECT GAIN
+12, +18dB
AVCC
GAIN
AD6600
FLT
GND
NOISE FILTER
AD6600 SUBCIRCUITS
Input Step Attenuator and Gain Stage
The AD6600 has two identical input attenuators, Channel A
and Channel B. These dual inputs are typically used as diversity
channels but may also process two independent IF signals. For
maximum oversampling the device is used in single channel mode;
in this case only one input channel is required. The attenuator
steps are 0 dB, –12 dB and –24 dB. The attenuator settings are
based on the decisions of the RSSI stage (see Peak Detector/
RSSI section). The outputs of the attenuators connect to an
analog multiplexer that selects either Channel A or B for subse-
quent processing (see Input Mode). The selected signal drives
a dual-gain amplifier set to either 12 dB or 18 dB; the selected
gain is also determined by the RSSI stage. Therefore, based on
all possible combinations of attenuation and gain, the input
signal receives –12 dB to +18 dB of voltage gain in 6 dB steps
(Table I). Overall gain-matching is typically within 0.1 dB. With
a bandwidth of 1 GHz, the phase delay through the front-end
ranges from 0.2 degrees to 0.5 degrees, depending on input
frequency. Additionally, the input impedance does not change
with attenuator settings so there is no AM-to-PM distortion.
ENCODE
630
–90
ENCODE
FLT
0
RESONANT
CONVERTER
ENC
4
PORT
TIMING
A/D
8
ENC
12
16
11
COMPLEMENT
20
RSSI
24 28 32 36 40 44 48 52 56 60
TWO'S
SNR – dB
DVCC
3
101
100
011
010
001
000
12dB SNR WINDOW
AB_OUT
D10–D0
RSSI [2:0]
CLK2
AD6600
101
100
011
010
001
000
0
–42
–60
–66
–72
–78
–84
–90
–96
–12
–18
–24
–30
–36
–48
–54