supply plane. If it is used for offset adjustment, the series
resistor and potentiometer should be located as close to the
converter as possible.
consistently done with a fixed series or parallel resistor. The
ADC700 can then be calibrated using the Gain and Offset
adjustments described in the calibration section. For ex-
ample, using the ±10V input range, one can decrease the
range slightly by paralleling the 10kΩ input resistor (pin 28
to pin 1) with a 610kΩ metal film resistor to achieve a
300µV LSB instead of the nominal standard 305.17578µV
binary LSB.
The Gain Adjust (pin 4) is an input that has a relatively high
input impedance and is susceptible to noise pickup. The
Gain Adjust pin should be bypassed with a 0.01µF to 0.1µF
capacitor whether or not the gain adjust feature is used.
If the 20V Analog input range is used (pin 28), the 10V
Range input (pin 27) may need to be shielded with ground
plane to reduce noise pickup.
OPTIONAL EXTERNAL GAIN AND OFFSET TRIM
Gain and Offset Error may be trimmed to zero using external
Gain and Offset trim potentiometers connected to the
ADC700 as shown in Figures 6 and Figure 7. A calibration
procedure in described in the Operating Instructions section.
ANALOG SIGNAL SOURCE IMPEDANCE
The input impedance of the ADC700, typical of most suc-
cessive approximation A/D converters, is relatively low
(2.5kΩ to 10kΩ). The input current of a successive
approximation A/D converter changes rapidly during the
conversion algorithm as each bit current is compared to the
analog input current. Since the output impedance of a
closed-loop amplifier or a sample-hold amplifier increases
with frequency and, in addition, the amplifier must settle to
the required accuracy in time for the next comparison/
decision after such a disturbance, care must be taken to
select the proper driving amplifier.
Multiturn potentiometers with 100ppm/°C or better TCR are
recommended for minimum drift over temperature. These
potentiometers may be any value from 10kΩ to 100kΩ. All
resistors should be 20% carbon or better. Pin 1 (Comparator
In) and pin 4 (Gain Adjust) may be left open if no external
adjustment is planned; however, pin 4 should always be
bypassed with 0.01µF or larger to Analog Common.
OPERATING INSTRUCTIONS
Unfortunately, high-accuracy operational amplifiers tend to
have low bandwidth, while wide-band amplifiers tend to
have lower accuracy. One solution is to use a wide-band but
perhaps less precise amplifier. Another solution is to add a
wide-band buffer amplifier such as the Burr-Brown OPA633
inside the feedback loop of a slower (but precision) ampli-
fier, Figure 5. This reduces the output impedance at high
frequencies yet preserves the accuracy at low frequencies.
When a sample/hold is needed, a high-linearity, high-speed
sample/hold such as the Burr-Brown SHC76 should be used
to drive the ADC700.
CALIBRATION
Offset and Gain may be trimmed by external Offset and
Gain potentiometers. Offset is adjusted first and then Gain.
Calibration values are listed in Table II for all ADC700
input ranges. Offset and Gain calibration can be accom-
plished to a precision of about ±1/2LSB using a static
adjustment procedure described below. By summing a small
sine or triangular wave voltage with the accurate calibration
voltage applied to the analog input, the output can be swept
through each of the calibration codes to more accurately
determine the transition points listed in Table II. NOTE: The
transition points are not the same as the code midpoints used
in the static calibration example.
ANALOG INPUT RANGES
The analog input circuits of the ADC700 can be connected
to accept unipolar or bipolar input signals. These ranges and
connections are tabulated in Table I. Circuit connections are
shown in Figures 6 and 7. Gain and offset adjustments are
described in the calibration section.
OFFSET ADJUSTMENT,
14-BIT RESOLUTION EXAMPLE
Static Adjustment Procedure (At Code Midpoints)
0V to +10V Range—Set the analog input to +1LSB14
=
To operate the ADC700 with a range that gives other
convenient values for the LSB, the input resistor may be
increased or decreased slightly without seriously affecting
the Gain Drift of the converter. Since the input resistors of
the ADC700 are within ±2% from unit to unit, this can be
0.00061V. Adjust the Offset potentiometer for a digital
output of 0004H. Set the analog input to +Full Scale –
2LSB14 = +9.9987V. Adjust the Gain potentiometer for a
digital output of FFFCH. For a half-scale calibration check,
set the analog input to +5.0000V and read a digital output
code of 8000H.
Precision
Low Bandwidth
Op Amp
INPUT
SIGNAL
RANGE
CONNECT
PIN 2
TO PIN
CONNECT
PIN 28
TO PIN
CONNECT
SIGNAL
TO PIN
OUTPUT CODE
Wideband
Buffer
BTCEN = 1 BTCEN = 0
A/D
Converter
±10V
±5V
±2.5V
0V to +5V
0V to +10V
0V to +20V
BOB
BOB
BOB
USB
USB
USB
BTC
BTC
BTC
—
—
—
1
1
1
26
26
26
Input Signal
Open
Pin 1
Pin 1
Open
28
27
27
27
27
28
OPA111
OPA27
OPA633
Analog
Common
Input Signal
FIGURE 5. Wideband Buffer Reduces Output Impedance at
High Frequencies.
TABLE I. ADC700 Input Range Connections.
®
ADC700
8
BB [ BURR-BROWN CORPORATION ]
