®

ADS7805

11

FIGURE 7. Full Scale Transfer Function.

REFERENCE

The ADS7805 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).

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 2.2

µF 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 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.

–9.99983V –9.9998V

–10V

Digital

Output

7FFF

Analog

Input

8000

9.999815V

+10V

Ideal Transfer Function

With External Resistors

Range of Transfer Function

Without External Resistors

9.9997V

–50mV

–15mV

CAP

CAP (pin 4) is the output of the internal reference buffer. A

2.2

µF 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 1

µF can cause the output buffer to oscillate and

may not have sufficient charge for the CDAC. Capacitor

values larger than 2.2

µF will have little affect on improving

performance.

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 ADS7805. 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.