AD9768 Datasheet, PDF (3/4 Pages) Analog Devices – Ultrahigh Speed IC D/A Converter

AD9768
THEORY OF OPERATION
Refer to the AD9768SD schematic.
The transistors pictured on the bottom of the diagram, con-
nected to paired transistors in the middle of the schematic, are
current sources which are always “on”. The paired transistors
are differential current switches, designed to steer current from
the current sources to either Pin 13 (I_O) or Pin 14 ( _IO ).
Digital inputs applied to Pins 1-8 determine which transistors
will be operating in each pair and establish what current will
flow at Pins 13 and 14.
speed, high performance device: optimum use requires careful
attention to all design details, including the layout of the circuit
in which the converter is used.
CONVENTIONAL AD9768SD
Refer to Figure 1, Conventional AD9768SD.
The output current of the AD9768 appears at Pin 13 (I_O) and
develops a voltage across the load resistor R_L which is based on:
A. I_M (the current flowing through the single-transistor
source discussed above)
The transistor on the extreme left of the schematic is a base
reference for the paired current switches and is used to assure
the switches will be centered around an ECL voltage swing. The
diodes connected to the base of this transistor are temperature
compensation devices for the base reference circuit.
B. Value of R_L
There are three different current sources in the AD9768 D/A.
The eight transistors shown on the bottom of the schematic are
structured as two identical groups of four current sources, each
of which is binarily weighted. The MSB group, comprised of the
four on the right, is connected to the LSB group through a 15:1
current divider made up of two 50 Ω and two 750 Ω resistor
networks. The geometry of the AD9768 guarantees the binary
weighing ratios among the 100, 200, 400 and 800 resistors in
each emitter circuit are correct.
The resistor values which are shown indicate the ratios among
the resistors, and not their nominal values.
The third current source is a single transistor, pictured in the
lower left portion of the schematic with its collector connected
to Pin 18 R_SET. Its function is to help establish the base voltage
on the eight current sources; it works in conjunction with the
external R_SET resistor selected by the user of the AD9768, and
the reference amplifier. Current flowing through this transistor
is referred to as I_M in the figures and text.
When the AD9768 is operating as a conventional current-output
D/A converter, I_M develops a voltage across R_SET which is one of
the inputs to the on-board reference amplifier shown in the
schematic. The other input to this amplifier is the on-chip
reference voltage of –1.26 volts.
The output of the reference amplifier adjusts the current-source
base reference voltage at Pin 17; this, in turn, adjusts the value
of I_M in the single-transistor current source and causes it to
develop a voltage across R_SET which maintains Pin 18 at the
–1.26 volts of the on-chip reference supply.
Figure 1. Conventional AD9768SD
I_M is a function of the return voltage (V_RET), the reference
voltage (V_REF), and the value of R_SET; all of these are selected by
the user for his application. The necessary equations for
calculating precise values for each are part of Figure 1. As
indicated, the voltage drop across R_L is added to the return
voltage; the resulting voltage is the total V_OUT of the converter.
VOLTAGE MULTIPLYING MODE
In addition to its use as an ultra-high speed current output D/A
converter, the AD9768 can also be used as a two-quadrant
multiplying D/A in either a voltage mode or a current mode.
Refer to Figure 2, Multiplying AD9768 (Voltage Mode).
When operating in this mode, the analog output of the AD9768
is influenced by the digital inputs and an external multiplying
voltage (V_M) applied to Pin 16 REFERENCE IN, which takes
the place of the internal reference used when the D/A is
operating in a conventional manner.
To maintain good stability in the internal loop reference
amplifier, a ceramic chip capacitor with a nominal value of
0.01 µF should be connected to Pin 17 COMPENSATION;
minimum recommended value for this capacitor is 3900 pF.
The temperature coefficient of the load resistor (R_L) can affect
the performance of the AD9768 D/A converter, as it can with
any current-output converter. The design and use of the
AD9768 and its dependence on an external R_SET resistor, how-
ever, make it sensitive also to the tempco of R_SET. The user is
cautioned to select R_L and R_SET resistors which have low tem-
perature coefficients.
DIGITAL GROUND (Pin 11) and ANALOG RETURN (Pin
12) are normally connected together; this connection should be
made as close as possible to the device case to minimize possible
noise problems. The AD9768 D/A is similar to any other high-
Figure 2. Multiplying AD9768 (Voltage Mode)
The value of I_M flowing through R_SET is set by the voltage of
V_RET minus the multiplying voltage (V_M), divided by R_SET; the
amount of this current is part of the equation which establishes
the analog output (V_OUT) of the AD9768 and is chosen by the
user for his application. As it is when operating the D/A in a
conventional fashion, V_RET can be any value between 0 volts and
+3 volts. V_M (for purposes of discussion here) is some negative
voltage and can be varied over a range which is approximately 1
volt peak-to-peak.
REV. A
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