Digital
Output
7FF
–10V
–9.99983V
–9.9998V
–50mV
Analog
Input
–15mV
9.9997V
9.999815V
+10V
Ideal Transfer Function
With External Resistors
Range of Transfer Function
Without External Resistors
800
FIGURE 7. Full Scale Transfer Function.
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.
REFERENCE
The ADS7804 can operate with its internal 2.5V reference or
an external reference. By applying an external reference to
pin 5, the internal reference can be bypassed. The reference
voltage at REF is buffered internally with the output on CAP
(pin 4).
CAP
CAP (pin 4) is the output of the internal reference buffer. A
2.2µF capacitor should be placed as close to the CAP pin as
possible to provide optimum switching currents for the CDAC
throughout the conversion cycle and compensation for the
output of the internal buffer. Using a capacitor any smaller
than 1µF can cause the output buffer to oscillate and may not
have sufficient charge for the CDAC. Capacitor values larger
than 2.2µF will have little affect on improving performance.
The internal reference has an 8 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).
REF
REF (pin 3) is an input for an external reference or the output
for the internal 2.5V reference. A 2.2µF 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
bandlimit noise on the reference. Using a smaller value
capacitor will introduce more noise to the reference degrad-
ing the SNR and SINAD. The REF pin should not be used to
drive external AC or DC loads.
The output of the buffer is capable of driving up to 2mA of
current to a DC load. DC loads requiring more than 2mA of
current from the CAP pin will begin to degrade the linearity
of the ADS7804. Using an external buffer will allow the
internal reference to be used for larger DC loads and AC
loads. Do not attempt to directly drive an AC load with the
output voltage on CAP. This will cause performance degra-
dation of the converter.
ADS7804
SBAS019A
9
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