SLVSL52 Data sheet

SLVSL52 September 2026 XTR100

ADVMIX

7.1.2 Basic Functionality in Loop-Powered Transmitters

The XTR1xx is interfaced to 4mA – 20mA loop as in Figure 7-1 which shows a simplified schematic omitting compensation. A power stage like wide V_IN LDO is required to power XTR1xx from the loop voltage V_LOOP. With INN input connected to ground, op amp positive input is kept at virtual ground and the user can derive the relation for I_LOOP:

Equation 1.

I_{1} = \frac{1}{R_{I N}} \times V_{I N P}

Equation 2.

V_{L -} = \frac{- R_{1}}{R_{I N}} \times V_{I N P}

Equation 3.

I_{2} = \frac{- 1}{R_{2}} \times V_{L -}

Equation 4.

I_{L O O P} = I_{1} + I_{2}

Equation 5.

I_{L o o p} = (1 + \frac{R_{1}}{R_{2}}) \times \frac{V_{I N P}}{R_{I N}}

Equation 6.

I_{L o o p} = \frac{R_{2} + R_{1}}{R_{I N}} \times \frac{V_{I N P}}{R_{2}}

Equation 7.

I_{L o o p} \approx \frac{R_{1}}{R_{I N}} \times \frac{V_{I N P}}{R_{2}} \approx {R a t i o}_{N O M} \times \frac{V_{I N P}}{R_{2}}, for R_{1} ≫ R_{2}

Given the target maximum loop current I_LOOP,max and the target maximum DAC output V_INP, the right R₂ is calculated then approximated to the nearest available standard resistor. With the actual R₂, the actual maximum DAC output is determined.

XTR100 Simplified Circuit for XTR100 in Loop-Powered Transmitter

Figure 7-1 Simplified Circuit for XTR100 in Loop-Powered Transmitter

Table 7-1 lists the proper standard R₂ resistor for certain DAC REF and target maximum I_LOOP.

Table 7-1 R2 Values for Different DAC Full-Scale and Different Target Maximum I_LOOP

		TARGET MAXIMUM OUTPUT CURRENT
		25mA		24mA		21mA
Reference	target FS	R2	R2_std	R2	R2_std	R2	R2_std
1.25V	1.24	12.40	12.4	12.92	12.9	14.76	14.7
2.5V	2.4	24.00	24	25.00	24.9	28.57	28.7
3V	2.8	28.00	28	29.17	29.1	33.33	33.2
3.3V	3	30.00	30	31.25	31.2	35.71	35.7