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LT6375

6375fa

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ERROR BUDGET ANALYSIS

Figure 4 shows the LT6375 in a current measurement

application. The error budget for this application is shown

in Table 8. The resistor divider ratio is set to 15 to divide

the 195V input common mode voltage down to 13V at the

op amp inputs. The 1A current and 10Ω sense resistor

produce an output full-scale voltage of 10V. Table 8 shows

theerrorsourcesinpartspermillion(ppm)ofthefull-scale

voltage across the temperature range of 25°C to 85°C.

Different sources of error contribute to the maximum ac-

curacy that can be achieved in an application. Gain error,

offset voltage and common mode rejection error combine

tosettheinitialerror.Additionally,thegainerrorandoffset

voltage drift across the temperature range. The excellent

gain accuracy, low offset voltage, high CMRR, low offset

voltage drift and low gain error drift of the LT6375 all

combine to enable extremely accurate measurements.

Over-The-Top OPERATION

When the input common mode voltage of the internal op

amp (VCMOP) in the LT6375 is biased near or above the V+

supply,theopampisoperatingintheOver-The-Topregion.

The op amp continues to operate with an input common

mode voltage of up to 76V above V– (regardless of the

positive power supply voltage V+), but its performance is

Table 8. Error Budget Analysis

ERROR SOURCE

LT6375A

LT6375

COMPETITOR 1

COMPETITOR 2

ERROR, ppm of FS

LT6375A LT6375 COMPETITOR 1 COMPETITOR 2

Accuracy, TA = 25°C

Initial Gain Error

0.0035% FS

0.006% FS

0.02% FS

0.03% FS

200

300

Offset Voltage

540µV

900µV

1100µV

500µV

110

Common Mode

195V/96dB =

3090µV

195V/89dB =

6920µV

195V/90dB =

6166µV

195V/86dB =

9770µV

309

692

617

977

Total Accuracy Error

398

842

927

1327

Temperature Drift

Gain

1ppm/°C ×60°C 1ppm/°C ×60°C

10ppm/°C ×60°C

600

Offset Voltage

16µV/°C ×60°C 22µV/°C ×60°C

15µV/°C ×60°C

10µV/°C ×60°C

132

Total Drift Error

156

192

690

660

Total Error

554

1034

1617

1987

APPLICATIONS INFORMATION

degraded. The op amp’s input bias currents change from

under±2nAto14µA.Theopamp’sinputoffsetcurrentrises

to ±50nA which adds ±9.5mV to the output offset voltage.

In addition, when operating in the Over-The-Top region,

the differential input impedance decreases from 1MΩ in

normaloperationtoapproximately 3.7kΩinOver-The-Top

operation. This resistance appears across the summing

nodes of the internal op amp and boosts noise and offset

while decreasing speed. Noise and offset will increase by

between 66% and 83% depending on the resistor divider

ratio setting. The bandwidth will be reduced by 40% to

45%. For more detail on Over-The-Top operation, consult

the LT6015 data sheet.

OUTPUT

The output of the LT6375 can typically swing to within

5mV of either rail with no load and is capable of sourcing

and sinking approximately 25mA. The LT6375 is internally

compensated to drive at least 1nF of capacitance under

any output loading conditions. For larger capacitive loads,

a 0.22µF capacitor in series with a 150Ω resistor between

the output and ground will compensate the amplifier to

drive capacitive loads greater than 1nF. Additionally, the

LT6375 has more gain and phase margin as the resistor

divider ratio is increased.

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