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1B21AN

Isolated, Loop-Powered Voltage-to-Current Converter

Analog Devices 

1B21

TC Considerations of External Resistors

The specifications for gain and offset temperature coefficient

(TC) for the IB21 excluded the effects of external components.

The total gain TC for the circuit in Figure 4 is

Gain TC = 1B21 Gain TC + (Tracking TC of RF and RI)

The offset TC is also affected by the thermal stability of the

internal voltage reference and its contribution is

Ref TC = (VREF)(RF/RO)(4 mA/V)(TC of VREF +

Tracking TC of RF and RO)/1 × 106

Total Offset TC = 1B21 Offset TC + Ref TC

Specifically using RF, RI and RO from Case 3 in Table 2, with

absolute TCs of ±25 ppm/°C

Gain TC = 50 + (25 + 25) = 100 ppm/°C

Offset TC = 300 + (6.225 V)(20k/124.3 k)(4 mA/V)

(20 + 25 + 25)/1 × 106 = ±580 nA/°C

Similarly, when using a resistor network with a tracking spec

of ±5 ppm/°C, the total gain TC is ±55 ppm/°C and the total

offset TC is ±400 nA/°C.

Rev. C | Page 6 of 8

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