AD8310-EVAL Analog Devices Inc, AD8310-EVAL Datasheet - Page 11

• Manufacturer Part Number: AD8310-EVAL
• Description: BOARD EVAL FOR AD8310
• Manufacturer: Analog Devices Inc

Specifications of AD8310-EVAL

• Rohs Status: RoHS non-compliant

PRODUCT OVERVIEW
The AD8310 has six main amplifier/limiter stages. These six
cells and their and associated g
handle the lower two-thirds of the dynamic range. Three top-
end detectors, placed at 14.3 dB taps on a passive attenuator,
handle the upper third of the 95 dB range. The first amplifier
stage provides a low noise spectral density (1.28 nV/√Hz).
Biasing for these cells is provided by two references: one
determines their gain, and the other is a band gap circuit that
determines the logarithmic slope and stabilizes it against supply
and temperature variations. The AD8310 can be enabled or
disabled by a CMOS-compatible level at ENBL (Pin 7).
The differential current-mode outputs of the nine detectors are
summed and then converted to single-sided form, nominally
scaled 2 μA/dB. The output voltage is developed by applying
this current to a 3 kΩ load resistor followed by a high speed
gain-of-four buffer amplifier, resulting in a logarithmic slope of
24 mV/dB (480 mV/decade) at VOUT (Pin 4). The unbuffered
voltage can be accessed at BFIN (Pin 6), allowing certain
functional modifications such as the addition of an external
postdemodulation filter capacitor and the alteration or
adjustment of slope and intercept.
COMMON
The last gain stage also includes an offset-sensing cell. This
generates a bipolarity output current, if the main signal path
exhibits an imbalance due to accumulated dc offsets. This
current is integrated by an on-chip capacitor that can be
increased in value by an off-chip component at OFLT (Pin 3).
The resulting voltage is used to null the offset at the output of
the first stage. Because it does not involve the signal input
connections, whose ac-coupling capacitors otherwise introduce
a second pole into the feedback path, the stability of the offset
correction loop is assured.
The AD8310 is built on an advanced, dielectrically isolated,
complementary bipolar process. In the following interface
diagrams shown in Figure 23 to Figure 26, resistors labeled as
R are thin-film resistors that have a low temperature coefficient
of resistance (TCR) and high linearity under large-signal
conditions. Their absolute tolerance is typically within ±20%.
SUPPLY
+INPUT
–INPUT
VPOS
INLO
INHI
1.0k
COMM
Ω
8mA
3
NINE DETECTOR CELLS
Figure 22. Main Features of the AD8310
COMPENSATION LOOP
AMPLIFIER STAGES
SIX 14.3dB 900MHz
SPACED 14.3dB
INPUT-OFFSET
BAND GAP REFERENCE
AND BIASING
m
styled full-wave detectors
2
MIRROR
2
/dB
μ
COMM
A
AD8310
3k
COMM
Ω
1k
+
–
Ω
COMM
3k
Ω
33pF
ENBL
OFLT
VOUT
BFIN
ENABLE
BUFFER
INPUT
OUTPUT
OFFSET
FILTER
Rev. F | Page 11 of 24
Similarly, capacitors labeled as C have a typical tolerance of
±15% and essentially zero temperature or voltage sensitivity.
Most interfaces have additional small junction capacitances
associated with them, due to active devices or ESD protection,
which might not be accurate or stable. Component numbering
in these interface diagrams is local.
ENABLE INTERFACE
The chip-enable interface is shown in Figure 23. The currents in
the diode-connected transistors control the turn-on and turn-off
states of the band gap reference and the bias generator. They are
a maximum of 100 μA when ENBL is taken to 5 V under worst-
case conditions. For voltages below 1 V, the AD8310 is disabled
and consumes a sleep current of less than 1 μA. When tied to the
supply or a voltage above 2 V, it is fully enabled. The internal
bias circuitry is very fast (typically <100 ns for either off or on).
In practice, however, the latency period before the log amp
exhibits its full dynamic range is more likely to be limited by
factors relating to the use of ac coupling at the input or the
settling of the offset-control loop (see the following sections).
INPUT INTERFACE
Figure 24 shows the essentials of the input interface. C
are parasitic capacitances, and C
capacitance, largely due to Q1 and Q2. In most applications,
both input pins are ac-coupled. The S switches close when
enable is asserted. When disabled, bias current I
the inputs float; therefore, the coupling capacitors remain
charged. If the log amp is disabled for long periods, small
leakage currents discharge these capacitors. Then, if they are
poorly matched, charging currents at power-up can generate a
transient input voltage that can block the lower reaches of the
dynamic range until it becomes much less than the signal.
A single-sided signal can be applied via a blocking capacitor to
either Pin 1 or Pin 8, with the other pin ac-coupled to ground.
Under these conditions, the largest input signal that can be
handled is 0 dBV (a sine amplitude of 1.4 V) when using a 3 V
supply; a 5 dBV input (2.5 V amplitude) can be handled with a
5 V supply. When using a fully balanced drive, this maximum
input level is permissible for supply voltages as low as 2.7 V.
Above 10 MHz, this is easily achieved using an LC matching
network. Such a network, having an inductor at the input,
usefully eliminates the input transient noted above.
ENBL
7
40kΩ
Figure 23. Enable Interface
COMM
2
D
is the differential input
TO BIAS
STAGES
AD8310
E
is shut off and
AD8310
P
and C
M