LTC1968 datasheet(23/28 Pages) LINER

23 / 28 page

LTC1968

1968f

The trade-off here is that on the one hand, the DC error is

input frequency dependent, so a calibration signal fre-

quency high enough to make the DC error negligible

should be used. On the other hand, as low a frequency as

can be used is best to avoid attenuation of the calibrated

AC signal, either from parasitic RC loading or insufficient

op amp gain. For instance, with a 1kHz calibration signal,

a 1MHz op amp will typically only have 60dB of open-loop

gain, so it could attenuate the calibration signal a full 0.1%.

AC-Only, 2 Point

The next most significant error for AC-coupled applica-

tions will be the effect of output offset voltage, noticeable

at the bottom end of the input scale. This too can be

calibrated out if two measurements are made, one with a

full-scale sine wave input and a second with a sine wave

input (of the same frequency) at 10% of full scale. The

trade-off in selecting this second level is that it should be

small enough that the gain error effect becomes small

compared to the gain error effect at full scale, while on the

other hand, not using so small an input that the input offset

voltage becomes an issue.

The calculations of the error terms for a 200mV full-scale

case are:

Gain =

Reading at 200mV – Reading at 20mV

180mV

Output Offset =

Reading at 20mV

Gain

–20mV

DC, 2 Point

DC-based calibration is preferable in many cases because

a DC voltage of known, good accuracy is easier to gener-

ate than such an AC calibration voltage. The only down

side is that the LTC1968 input offset voltage plays a role.

It is therefore suggested that a DC-based calibration

scheme check at least two points: ±full scale. Applying the

–full-scale input can be done by physically inverting the

voltage or by applying the same +full-scale input to the

opposite LTC1968 input.

For an otherwise AC-coupled application, only the gain

term may be worth correcting for, but for DC-coupled

applications, the input offset voltage can also be calcu-

lated and corrected for.

The calculations of the error terms for a 200mV full-scale

case are:

Gain =

Reading at 200mV + Reading at – 200mV

400mV

Input Offset =

Reading at – 200mV – Reading at 200mV

2 •Gain

Note: Calculation of and correction for input offset voltage

are the only way in which the two LTC1968 inputs (IN1,

IN2) are distinguishable from each other. The calculation

above assumes the standard definition of offset; that a

positive offset is the case of a positive voltage error inside

the device that must be corrected by applying a like

negative voltage outside. The offset is referred to which-

ever pin is driven positive for the +full-scale reading.

DC, 3 Point

One more point is needed with a DC calibration scheme to

determine output offset voltage: +10% of full scale.

The calculation of the input offset is the same as for the

2-point calibration above, while the gain and output offset

are calculated for a 200mV full-scale case as:

Gain =

Reading at 200mV – Reading at 20mV

180mV

Output Offset =

Reading at 200mV +Reading at – 200mV – 400mV • Gain

APPLICATIO S I FOR ATIO

LTC1968 датащи(PDF) 23 Page - Linear Technology

LTC1968 датащи(HTML) 23 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

датащи скачать

ссылки URL