DAC161S997RGHT datasheet(12/27 Pages) TI

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DAC

IDAC

IAUX

ILOOP

LOOP+

LOOP-

R1 = 80k

BASE

OUT

COMA

DAC161S997

VLOCAL

R2 = 40

DAC

IDAC

IAUX

ILOOP

LOOP+

LOOP-

R1 = 80k

BASE

OUT

COMA

R2 = 40

DAC161S997

SNAS621 – JUNE 2013

www.ti.com

APPLICATION INFORMATION

16-BIT DAC AND LOOP DRIVE

DC Characteristics

The DAC converts the 16-bit input code in the DACCODE registers to an equivalent current output. The

ΣΔ DAC

output is a current pulse which is then filtered by a third-order RC lowpass filter and boosted to produce the loop

current (ILOOP) at the device OUT pin.

Figure 9. Loop-Powered Transmitter

Figure 9 shows the principle of operation of the DAC161S997 in the Loop-Powered Transmitter (the circuit

details are omitted for clarity). In Figure 9, ID and IA represent supply (quiescent) currents of the internal digital

and analog blocks. IAUX represents supply (quiescent) current of companion devices present in the system, such

as the voltage regulator and the digital interface. Because both the control loop formed by the amplifier and the

bipolar transistor force the voltage across R1 and R2 to be equal, under normal conditions, the ILOOP is dependent

only on IDAC through the following relationship (see Equation 1).

ILOOP = (1 + R1 / R2) IDAC

where

•

IDAC = ƒ(DACCODE)

(1)

Although ILoop has a number of component currents, ILOOP = IDAC + ID + IA + IAUX + IE, only IE is regulated by the

loop to maintain the relationship shown in Equation 1. Because only the magnitude of IE is controlled, not the

direction, there is a lower limit to ILOOP. This limit is dependent on the fixed components IA and ID, and on system

implementation through IAUX.

Figure 10. Self-Powered Transmitter

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