AD7869
–6–
CONVERTER DETAILS
The AD7869 is a complete 14-bit I/O port; the only external
components required for normal operation are pull-up resistors
for the ADC data outputs, and power supply decoupling capaci-
tors. The AD7869 is comprised of a 14-bit successive approxi-
mation ADC with a track/hold amplifier, a 14-bit DAC with a
buffered output and two 3 V buried Zener references, a clock os-
cillator and control logic.
ADC CLOCK
The AD7869 has an internal clock oscillator that can be used for
the ADC conversion procedure. The oscillator is enabled by ty-
ing the CLK input to V
SS
. The oscillator is laser trimmed at the
factory to give a maximum conversion time of 10 µs. The mark/
space ratio can vary from 40/60 to 60/40. Alternatively, an exter-
nal TTL compatible clock may be applied to this input. The al-
lowable mark/space ratio of an external clock is 40/60 to 60/40.
RCLK is a clock output, used for the serial interface. This out-
put is derived directly from the ADC clock source and can be
switched off at the end of conversion with the CONTROL
input.
ADC CONVERSION TIMING
The conversion time for both external clock and continuous in-
ternal clock can vary from 19 to 20 rising clock edges, depending
on the conversion start to ADC clock synchronization. If a con-
version is initiated within 30 ns prior to a rising edge of the ADC
clock, the conversion time will consist of 20 rising clock edges,
i.e., 9.5 µs conversion time. For noncontinuous internal clock,
the conversion time always consists of 19 rising clock edges.
ADC TRACK-AND-HOLD AMPLIFIER
The track-and-hold amplifier on the analog input of the AD7869
allows the ADC to accurately convert an input sine wave of 6 V
peak–peak amplitude to 14-bit accuracy. The input impedance is
typically 9 kΩ; an equivalent circuit is shown in Figure 1. The
input bandwidth of the track/hold amplifier is much greater
than the Nyquist rate of the ADC even when the ADC is oper-
ated at its maximum throughput rate. The 0.1 dB cutoff fre-
quency occurs typically at 500 kHz. The track/hold amplifier
acquires an input signal to 14-bit accuracy in less than 2 µs. The
overall throughput rate is equal to the conversion time plus the
track/hold amplifier acquisition time. For a 2.0 MHz input clock,
the throughput time is 12 µs max.
AD7869*
4.5kΩ
4.5kΩ
*ADDITIONAL PINS OMITTED FOR CLARITY
V
IN
TO INTERNAL
COMPARATOR
TRACK/HOLD
AMPLIFIER
TO INTERNAL
3V REFERENCE
Figure 1. ADC Analog Input
The operation of the track/hold amplifier is essentially transpar-
ent to the user. The track/hold amplifier goes from its track
mode to its hold mode at the start of conversion on the rising
edge of
CONVST.
INTERNAL REFERENCES
The AD7869 has two on-chip temperature compensated buried
Zener references that are factory trimmed to 3 V ±10 mV. One
reference provides the appropriate biasing for the ADC, while
the other is available as a reference for the DAC. Both reference
outputs are available (labelled RO DAC and RO ADC) and are
capable of providing up to 500 µA to an external load.
The DAC input reference (RI DAC) can be sourced externally
or connected to any of the two on-chip references. Applications
requiring good full-scale error matching between the DAC and
the ADC should use the ADC reference as shown in Figure 4.
The maximum recommended capacitance on either of the refer-
ence output pins for normal operation is 50 pF. If either of the
reference outputs is required to drive a capacitive load greater
than 50 pF, then a 200 Ω resistor must be placed in series with
the capacitive load. The addition of decoupling capacitors,
10 µF in parallel with 0.1 µF as shown in Figure 2, improves
noise performance. The improvement in noise performance can
be seen from the graph in Figure 3. Note: this applies for the
DAC output only; reference decoupling components do not af-
fect ADC performance. Consequently, a typical application will
have just the DAC reference decoupled with the other one open
circuited.
RI DAC
200Ω
RO DAC
or
RO ADC*
EXT LOAD
GREATER THAN 50pF
*RO DAC/RO ADC CAN BE LEFT
OPEN CIRCUIT IF NOT USED
10µF
0.1µF
Figure 2. Reference Decoupling Components
DAC OUTPUT AMPLIFIER
The output from the voltage mode DAC is buffered by a non-
inverting amplifier. The buffer amplifier is capable of developing
±3 V across 2 kΩ and 100 pF load to ground and can produce
6 V peak-to-peak sine wave signals to a frequency of 20 kHz.
The output is updated on the falling edge of the LDAC input.
The output voltage settling time, to within 1/2 LSB of its final
value, is typically less than 3.5 µs.
The small signal (200 mV p–p) bandwidth of the output buffer
amplifier is typically 1 MHz. The output noise from the ampli-
fier is low with a figure of 30 nV/√
Hz at a frequency of 1 kHz.
The broadband noise from the amplifier exhibits a typical peak-
to-peak figure of 150 µV for a 1 MHz output bandwidth. Figure
3 shows a typical plot of noise spectral density versus frequency
for the output buffer amplifier and for either of the on-chip
references.
