AD6140
FREQUENCY PLAN
otherwise, the expected gain will not be obtained from the
AD6140. In addition to the mixer, there is a mixer post-
amplifier within the AD6140. The total gain from the mixer
and mixer post-amplifier is 5 dB.
The AD6140 and its Σ∆ modulator are designed for a specific
frequency plan: a 6.144 MHz master clock, a 49.6 MHz first
IF input, and a 192 kHz center frequency in the bandpass Σ∆
modulator. The local oscillator may use high-side or low-side
injection. The specifications for the AD6140 are only valid for
this frequency plan. Any deviation from this frequency plan may
result in degradation of the specified performance. Furthermore,
there are only specific frequency plans which will result in ac-
ceptable performance for most applications. To avoid problems,
do not change the frequency plan.
The Σ∆ modulator uses a 6.144 MHz clock, which is a differen-
tial ECL input. There is an ECL-to-CMOS converter on the
AD6140, which converts this differential ECL input into a
single-ended CMOS signal. This 6.144 MHz single-ended CMOS
clock is provided at Pin 7 (Σ∆_CLOCK_OUT). The output
data of the AD6140 is a 6.144 MHz single bitstream at Pin 6
(Σ∆_DATA_OUT). The signal gain through the Σ∆ modulator
is –0.77 dB.
USING THE AD6140
Within the Σ∆ modulator, the data output digital bitstream is
fed through a 1-bit D/A converter and is fed back to numerous
internal points. The level of this feedback signal, known as the
full-scale level, defines the Σ∆ modulator input signal level,
which would result in the output digital bitstream containing the
maximum number of ones possible. This condition, known as
maximum ones density, represents the maximum in-band out-
put signal power of the Σ∆ modulator. The full-scale level is set
to 2 V p-p or –4.77 dBm (relative to 1500 Ω). However, if a
signal into the modulator is –4.77 dBm, the modulator will
enter an unstable state. Consequently, the maximum input to
the modulator is constrained to 5 dB less than the signal, which
would produce maximum ones density. This level, defined as
the clip level, is –9.77 dBm (relative to 1500 Ω).
In this section, we will examine a few areas of special impor-
tance and include a few general applications tips. As is true of
any device operating in the IF frequency range, special care
must be taken in PC board layout. The location of the particular
grounding points must be considered, with the objective of
minimizing any unwanted signal coupling. Specifically, care
should be taken in the layout of the IF and LO signal paths as
well as the data and clock digital bit-streams. Layout of these
portions of the PC board require special attention in order to
ensure that the high frequency portions of these signals do not
couple into other signals in the system. In order to maintain
balance in differential signal levels, be sure to keep short and
equal length transmission lines.
The power supplies should be decoupled to ensure a clean dc
signal. Special care should be taken with respect to ensuring that
the BUFFER_VDD is especially clean and at the appropriate
levels since the output in-band noise floor is particularly sensi-
tive to this supply.
The maximum signal into the modulator does not correspond to
maximum ones density. The entire dynamic range of the result-
ing analog to digital converter (Σ∆ modulator plus decimation
filter) is not realized. In order to relate the maximum signal into
the modulator to the maximum signal out of the modulator, a
gain of 5 dB should be applied in the decimation filter.
The IF input signal should be impedance matched and ac
coupled. The impedance looking into the IF input pin is typi-
cally a 2.5 kΩ resistance in parallel with a 12 pF capacitance.
The 1 V reference signal should be regulated and filtered.
As can be seen in Figure 5, the output signal to noise ratio will
increase until a point at which it rapidly degrades. This point
represents the input signal level where the Σ∆ modulator has
become unstable. As a result, the maximum input signal level is
constrained by the point at which it is so high that instability
occurs in the modulator. Dynamic range is defined as the differ-
ence between the integrated noise floor (within a particular
bandwidth) and the power in the output signal just before the
Σ∆ modulator has become unstable. For a typical 6.25 kHz
bandwidth centered around 192 kHz, the AD6140 has 83 dB of
dynamic range.
The value of the BIAS_RESISTOR (Pin 16) is 39 kΩ. The bias
resistor sets the current consumption of the AD6140. Because
the AD6140 was characterized with a 39 kΩ bias resistor, this is
the only value for which the AD6140 specifications are guaran-
teed. Maximum current consumption is measured when the
AD6140 is operating at maximum gain.
The AGC integration capacitor should be large enough to by-
pass any externally-generated noise on the internal AGC line to
ground in addition to providing a path for the charging and
discharging of the AGC current. In the Motorola ReFLEX
chipset solution, this capacitor is 0.1 µF. The AGC time con-
stant is switch-selectable with the AGC_TC_SELECT pin (Pin
10). The AGC time constant has a typical current ratio of 56:1
when in the fast mode relative to slow mode. The nominal
AGC current in the fast (high current) position is 2.8 µA and
in the slow (low current) position is 50 nA. The AGC time
constant may be calculated from Equation 1.
In order to increase the range of useful input signals of the
AD6140, an AGC detector is employed which senses the input
signal level to the Σ∆ modulator and adjusts the gain in the pream-
plifier. The AGC circuitry provides 13 dB of automatic gain
control range. The AGC operates when the internal AGC voltage
is between 700 mV (minimum gain) and 1.55 V (maximum gain).
This voltage can be measured on the AGC_CAPACITOR pin
(Pin 13).
The AD6140 can be configured with the chip powered up or
down. In order to power the chip down, set pin POWER_DOWN
(Pin 9) high. In order to power it up, set pin POWER_DOWN
(Pin 9) low.
CV
I
T =
(1)
where T is the AGC time constant in seconds, C is the value of
the AGC capacitor in Farads, V is the full-scale change in the
AGC voltage, and I is the charging current in amperes.
Finally, an auxiliary amplifier used for biasing an external dis-
crete LNA is provided with the AD6140.
REV. 0
–7–
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