AD7870A
CONVERTER DETAILS
The AD7870A is a complete 12-bit A/D converter, requiring no
external components apart from power supply decoupling ca-
pacitors. It is comprised of a 12-bit successive approximation
ADC based on a fast settling voltage-output DAC, a high speed
comparator and SAR, a track/hold amplifier, a 3 V buried Zener
reference, a clock oscillator and control logic.
INTERNAL REFERENCE
The AD7870A has an on-chip temperature compensated buried
Zener reference that is factory trimmed to 3 V ± 10 mV. In-
ternally it provides both the DAC reference and the dc bias re-
quired for bipolar operation. The reference output is available
(REF OUT) and capable of providing up to 500 µA to an ex-
ternal load.
Figure 4. Analog Input
ANALOG INPUT
Figure 4 shows the AD7870A analog input. The analog input
range is ±3 V into an input resistance of typically 15 kΩ. The
designed code transitions occur midway between successive
integer LSB values (i.e., 1/2 LSB, 3/2 LSBs, 5/2 LSBs . . .
FS 3/2 LSBs). The output code is twos complement binary with
1 LSB = FS/4096 = 6 V/4096 = 1.46 mV. The ideal input/out-
put transfer function is shown in Figure 5.
The maximum recommended capacitance on REF OUT for
normal operation is 50 pF. If the reference is required for use
external to the AD7870A, it should be decoupled with a 200 Ω
resistor in series with a parallel combination of a 10 µF tantalum
capacitor and a 0.1 µF ceramic capacitor. These decoupling
components are required to remove voltage spikes caused by the
AD7870A’s internal operation.
Figure 3. Reference Circuit
Figure 5. Bipolar Input/Output Transfer Function
TRACK-AND-HOLD AMPLIFIER
The track-and-hold amplifier on the analog input of the AD7870A
allows the ADC to accurately convert an input sine wave of 6 V
peak-peak amplitude to 12-bit accuracy. The input bandwidth
of the track/hold amplifier is much greater than the Nyquist rate
of the ADC even when the ADC is operated at its maximum
throughput rate. The 0.1 dB cutoff frequency occurs typically at
500 kHz. The track/hold amplifier acquires an input signal to
12-bit accuracy in less than 2 µs. The overall throughput rate is
equal to the conversion time plus the track/hold amplifier ac-
quisition time. For a 2.5 MHz input clock the throughput rate
is 10 µs max.
BIPOLAR OFFSET AND FULL SCALE ADJUSTMENT
In most digital signal processing (DSP) applications, offset and
full-scale errors have little or no effect on system performance.
Offset error can always be eliminated in the analog domain by
ac coupling. Full-scale error effect is linear and does not cause
problems as long as the input signal is within the full dynamic
range of the ADC. Some applications will require that the input
signal span the full analog input dynamic range. In such applica-
tions, offset and full-scale error will have to be adjusted to zero.
Where adjustment is required, offset error must be adjusted be-
fore full-scale error. This is achieved by trimming the offset of
the op amp driving the analog input of the AD7870A while the
input voltage is 1/2 LSB below ground. The trim procedure is as
follows: apply a voltage of 0.73 mV (–1/2 LSB) at V1 in Figure 6
and adjust the op amp offset voltage until the ADC output code
flickers between 1111 1111 1111 and 0000 0000 0000. Gain
error can be adjusted at either the first code transition (ADC
negative full scale) or the last code transition (ADC positive full
scale). The trim procedures for both cases are as follows (see
Figure 6).
The operation of the track/hold is essentially transparent to the
user. The track/hold amplifier goes from its tracking mode to its
hold mode at the start of conversion. The track-to-hold transi-
tion occurs on the falling edge of CONVST.
REV. 0
–6–
ADI [ ADI ]
