TDA8785 Datasheet, PDF (6/24 Pages) NXP Semiconductors – 8-bit high-speed analog-to-digital converter with gain and offset controls

Philips Semiconductors
8-bit high-speed analog-to-digital converter
with gain and offset controls
Product specification
TDA8785
FUNCTIONAL DESCRIPTION
The TDA8785 is composed of an 8-bit ADC (30 Msps),
a wide-band gain amplifier, an input offset amplifier and an
8-bit dynamic adjustment DAC.
Input signal
Two input pins are provided to apply a differential input
signal with a wide range (100 to 1000 mV differential).
It is also possible to apply a single signal by setting a DC
voltage on one of the differential pins and supplying the
signal to the other.
Controlled gain amplifier
The gain amplifier is used to adjust the wide input signal
range to the fixed ADC input range of 1 V (p-p).
A large gain of 20 dB can be achieved with low-noise
behaviour and a large bandwidth of 100 MHz to correctly
amplify square type signals with step edges. Using pin 6,
it is possible to reduce the internal bandwidth of the gain
amplifier via an external capacitor and thus improve its
noise behaviour. The gain amplifier is controlled via an
external differential voltage (single input can also be
applied).
Input offset amplifier and adjustment DAC
The Input offset amplifier contains two different control
inputs (which can also be single):
• Slow offset control, for slow variation characteristics
(e.g. temperature, supply voltage, etc.)
• Fast offset control, for correction related to the clock
rate.
Slow offset control is carried out by an external voltage
while fast offset control is digitally carried out via the
internal 8-bit DAC with external connections of the
respective pins V_DACO(n), V_DACO(p), V_FOFF(n) and V_FOFF(p).
The internal 8-bit DAC operates at the ADC clock rate to
allow dynamic corrections on the input signal chain based
on the signal processing information carried out after the
digital conversion. The output voltage amplitude of the
DAC can be controlled via a different input voltage (which
can also be single) in a range of ±25% with a 150 Ω DAC
output load.
The DAC can also be used for the gain or the slow offset
control with some external DC voltage adaptations and
can be considered as a separate function of the ADC
chain. The DAC can be used independently, for example
as a video DAC.
8-bit ADC
The 8-bit ADC converts a signal of 1 V (p-p) from the
controlled gain amplifier into an 8-bit coded digital word at
a maximum rate of 30 Msps. Its reference voltage is
supplied by the general voltage regulator. The output data
format can either be binary, two’s complement or 3-state
by selecting pin OF.
When all the differential inputs on the offset amplifier
(V_SOFF(p), V_SOFF(n), V_FOFF(n), V_FOFF(p), V_i(p) and V_i(n)) are at
0 V (equivalent to both inputs short-circuited), the output
code of the ADC is code 8.
Internal voltage regulator
An internal voltage regulator provides all the references for
the different blocks. A stable 2.75 V voltage reference
output is provided for use in the application environment.
One application is to connect all the slow control inputs
(V_FSDAC(p), V_FSDAC(n), V_SOFF(p), V_SOFF(n), V_FSAD(p) and
V_FSAD(n)) to this reference, either to their two differential
inputs to get the nominal settings or to one of the
differential inputs to have easy single-input control.
All these control inputs have the same control range.
1997 Dec 18
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