10

®

ADS7819

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.

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.

tion. However, larger resistances will push the useful adjust-

probably not exceed 20k

Ω and P2 100kΩ in order to main-

tain reasonable sensitivity.

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 50

Ω 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 external 50

Ω 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 50

Ω

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 50

Ω resistor in

the circuit.

REFERENCE

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.)

REF

REF (pin 3) is an input for an external reference or the

output for the internal 2.5V reference. A 0.1

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 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.

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.

FIGURE 5. Circuit Diagram With and Without External Resistors.

–2.46V

–2.5V

Digital

Output

+ Full Scale

Analog

Input

– Full Scale

Typical Transfer Function

with 50

Ω Resistor

Typical Transfer Function

Without 50

Ω Resistor

2.46V

2.5V