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PDF TDA8780M Data sheet ( Hoja de datos )
| Número de pieza | TDA8780M | |
| Descripción | True logarithmic amplifier | |
| Fabricantes | NXP Semiconductors | |
| Logotipo |  |
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INTEGRATED CIRCUITS
DATA SHEET
TDA8780M
True logarithmic amplifier
Product specification
Supersedes data of November 1994
File under Integrated Circuits, IC03
1995 Jul 25
1 page  
Philips Semiconductors
True logarithmic amplifier
Product specification
TDA8780M
FUNCTIONAL DESCRIPTION
A true logarithmic amplifier can be realized from a cascade
of similar stages each stage consisting of a pair of
amplifiers whose inputs and outputs are connected in
parallel. One of these amplifiers can be formed by an
undegenerated long-tailed pair which provides high gain
but limited linear input signal-handling capability. The
other amplifier can be formed by a degenerated long-tailed
pair which provides a gain of unity and a much larger linear
input signal-handling capability.
The overall cascade amplifies very small input signals but,
once these reach the level at which the undegenerated
long-tailed pair in the last stage is at the limit of its linear
signal-handling capability, the output voltage becomes
logarithmically dependent on the input signal level. This
behaviour continues until the input signal reaches the level
at which undegenerated long-tailed pair in the first stage is
at the limit of its linear input signal-handling capability. The
transfer characteristic beyond this point then depends on
the exact configuration of the degenerated long-tailed pair
in the first stage.
Five stages are used in the TDA8780M to provide a 72 dB
true logarithmic dynamic range. The DC bias current in the
undegenerated long-tailed pair in the first stage is made
externally adjustable, using an off-chip resistor, to provide
a small-signal gain adjustment facility. The small signal
gain defined by this resistor is valid when the IC is
operating in the “linear” mode, for input signals typically
less than 60 µV.
A high-level limiter is inserted between the first and second
stages to provide a constant limiting output voltage which
is essentially independent of the value of the gain setting
resistor. These stages can be driven by single-ended or
differential inputs. The DC operating point is set by overall
on-chip feedback decoupled by two off-chip capacitors
which define the low-frequency cut-off point. The
performance is stabilized against temperature and DC
power supply variations. The input to the true logarithmic
amplifier is protected against damage due to excessive
differential input signals by diodes.
The differential output from the true logarithmic amplifier is
converted internally to a single-ended output by an on-chip
operational amplifier arrangement in which the DC output
level is set by an externally-supplied reference voltage.
The output is capable of driving loads down to 10 kΩ. The
limiting output voltage and the output drive capability have
been chosen to facilitate interfacing to analog-to-digital
converters. A major part of the DC power supply current
consumption of the device is associated with provision of
this output drive capability. The DC power supply
consumption is significantly less when the device is driving
smaller loads.
A power-down facility allows the circuit to be disabled from
a TTL-level compatible control input.
1995 Jul 25
5
5 Page 
Philips Semiconductors
True logarithmic amplifier
Product specification
TDA8780M
SOLDERING
Introduction
There is no soldering method that is ideal for all IC
packages. Wave soldering is often preferred when
through-hole and surface mounted components are mixed
on one printed-circuit board. However, wave soldering is
not always suitable for surface mounted ICs, or for
printed-circuits with high population densities. In these
situations reflow soldering is often used.
This text gives a very brief insight to a complex technology.
A more in-depth account of soldering ICs can be found in
our “IC Package Databook” (order code 9398 652 90011).
Reflow soldering
Reflow soldering techniques are suitable for all SO
packages.
Reflow soldering requires solder paste (a suspension of
fine solder particles, flux and binding agent) to be applied
to the printed-circuit board by screen printing, stencilling or
pressure-syringe dispensing before package placement.
Several techniques exist for reflowing; for example,
thermal conduction by heated belt. Dwell times vary
between 50 and 300 seconds depending on heating
method. Typical reflow temperatures range from
215 to 250 °C.
Preheating is necessary to dry the paste and evaporate
the binding agent. Preheating duration: 45 minutes at
45 °C.
During placement and before soldering, the package must
be fixed with a droplet of adhesive. The adhesive can be
applied by screen printing, pin transfer or syringe
dispensing. The package can be soldered after the
adhesive is cured.
Maximum permissible solder temperature is 260 °C, and
maximum duration of package immersion in solder is
10 seconds, if cooled to less than 150 °C within
6 seconds. Typical dwell time is 4 seconds at 250 °C.
A mildly-activated flux will eliminate the need for removal
of corrosive residues in most applications.
Repairing soldered joints
Fix the component by first soldering two diagonally-
opposite end leads. Use only a low voltage soldering iron
(less than 24 V) applied to the flat part of the lead. Contact
time must be limited to 10 seconds at up to 300 °C. When
using a dedicated tool, all other leads can be soldered in
one operation within 2 to 5 seconds between
270 and 320 °C.
Wave soldering
Wave soldering techniques can be used for all SO
packages if the following conditions are observed:
• A double-wave (a turbulent wave with high upward
pressure followed by a smooth laminar wave) soldering
technique should be used.
• The longitudinal axis of the package footprint must be
parallel to the solder flow.
• The package footprint must incorporate solder thieves at
the downstream end.
1995 Jul 25
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet TDA8780M.PDF ] | |
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