The external 50Ω resistor shown in Figure 4b may not be
necessary in some applications. This resistor provides trim
capability for the offset and compensates for a slight gain
adjustment internal to the ADS7819. Not using the 50Ω
resistor will cause a small gain error but will have no effect
on the inherent offset error. Figure 5 shows typical transfer
function characteristics with and without the 50Ω resistor in
the circuit.
CALIBRATION
The ADS7819 can be trimmed in hardware or software. The
offset should be trimmed before the gain since the offset
directly affects the gain.
Hardware Calibration
To calibrate the offset and gain of the ADS7819, install the
proper resistors and potentiometers as shown in Figure 4a.
The calibration range is ±12mV for bipolar zero and ±30mV
for full scale.
REFERENCE
Potentiometer P1 and resistor R1 form the offset adjust
circuit and P2 and R2 the gain adjust circuit. The exact values
are not critical. R1 and R2 should not be made any larger than
the value shown. They can easily be made smaller to provide
increased adjustment range. Reducing these below 15% of
the indicated values could begin to adversely affect the
operation of the converter.
The ADS7819 can operate with its internal 2.5V reference or an
external reference. By applying an external reference to pin 3,
the internal reference can be bypassed. The reference voltage at
REF is buffered internally and output on CAP (pin 4).
The internal reference has a 6 ppm/°C drift (typical) and
accounts for approximately 20% of the full scale error
(FSE = ±0.5% for low grade, ±0.25% for high grade.)
P1 and P2 can also be made larger to reduce power dissipa-
tion. However, larger resistances will push the useful adjust-
ment range to the edges of the potentiometer. P1 should
probably not exceed 20kΩ and P2 100kΩ in order to main-
tain reasonable sensitivity.
REF
REF (pin 3) is an input for an external reference or the
output for the internal 2.5V reference. A 0.1µF capacitor
should be connected as close to the REF pin as possible. The
capacitor and the output resistance of REF create a low pass
filter to band limit noise on the reference. Using a smaller
value capacitor will introduce more noise to the reference
degrading the SNR and SINAD. The REF pin should not be
used to drive external AC or DC loads.
Software Calibration
To calibrate the offset and gain of the ADS7819, no external
resistors are required. See the No Calibration section for
details on the effects of the external resistor.
No Calibration
See Figure 4b for circuit connections. Note that the actual
voltage dropped across the 50Ω resistor is nearly two orders
of magnitude lower than the voltage dropped across the
internal resistor divider network. This should be taken into
consideration when choosing the accuracy and drift specifi-
cations of the external resistors. In most applications, 1%
metal-film resistors will be sufficient.
The range for the external reference is 2.3V to 2.7V and
determines the actual LSB size. Increasing the reference
voltage will increase the full scale range and the LSB size
of the converter which can improve the SNR.
Digital
Output
+ Full Scale
–2.5V –2.46V
Analog
Input
2.5V
2.46V
Typical Transfer Function
with 50Ω Resistor
Typical Transfer Function
Without 50Ω Resistor
– Full Scale
FIGURE 5. Circuit Diagram With and Without External Resistors.
®
10
ADS7819
BB [ BURR-BROWN CORPORATION ]
