DAC161S997RGHT datasheet(14/27 Pages) TI

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LOOP+

LOOP-

A(s)Gm

IDAC

IAUX

ILOOP

A(s)Gmve

LOOP+

LOOP-

DAC161S997

BASE

OUT

COMA

RX1

CX1

CX2

CX3

CX4

A(s)

IAUX

IDAC

DAC161S997

SNAS621 – JUNE 2013

www.ti.com

Loop Compliance

The maximum V(LOOP+,LOOP-) potential is limited by the choice of step-down regulator, and the external BJT’s

Collector Emitter breakdown voltage. For minimum V(LOOP+, LOOP

−) potential consider TROUBLEFigure 10.

Here, observe that V(LOOP+,LOOP

−) ≅ min(VCE) + ILOOPRE + ILOOPR2 = min(VCE) + 0.53V + 0.96V = 3.66V, at

ILOOP = 24mA. The voltage drop across internal R2 is specified in ELECTRICAL CHARACTERISTICS.

AC Characteristics

The approximate frequency dependent characteristics of the loop drive circuit can be analyzed using the circuit in

Figure 12:

Figure 12. Capacitances Affecting Control Loop

Here it is assumed that the internal amplifier dominates the frequency response of the system, and it has a single

pole response. The BJT’s response, in the bandwidth of the control loop, is assumed to be frequency

independent and is characterized by the transconductance gm and the output resistance ro.

As in previous sections IDAC and IAUX represent the filtered output of the ∑Δ modulator and the quiescent current

of the companion devices.

The circuit in Figure 12 can be further simplified by omitting the on-board capacitances, whose effect will be

discussed in Stability, and by combining the amplifier, the external transistor and resistor RE into one Gm block.

The resulting circuit is shown in Figure 13.

By assuming that the BJT’s output resistance (ro) is large, the loop current ILOOP can be expressed as:

(3)

Figure 13. AC Analysis Model of a Transmitter

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