DAC161S997RGHT datasheet(16/27 Pages) TI

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DAC161S997

SNAS621 – JUNE 2013

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The output impedance of the transmitter is a product of the external BJT's output resistance ro, and the frequency

characteristics of the internal amplifier. At low frequencies this results in a large impedance that does not

significantly affect the output current accuracy.

PSRR

Power Supply Rejection Ratio is defined as the ability of the current control loop to reject the variations in the

supply current of the companion devices, IAUX. Specifically:

(12)

It was shown in AC Characteristics that the IAUX affects ILOOP via the high-pass path whose corner frequency is

dependent on the effective Gm of the external BJT. If that dependence were not mitigated with the degenerating

resistor RE, the PSRR would be degraded at low output current ILOOP.

The typical PSRR performance of the transmitter shown in Figure 7 is shown in TYPICAL PERFORMANCE

CHARACTERISTICS.

Stability

The current control loop's stability is affected by the impedances present in the system. Figure 12 shows the

simplified diagram of the control loop, formed by the on-board amplifier and an external BJT, and the lumped

capacitances CX1 through CX4 that model any other external elements.

CX1 typically represents a local step-down regulator, or LDO, and any other companion devices powered from the

LOOP+. This capacitance reduces the stability margins of the control loop, and therefore it should be limited.

RX1 can be used to isolate CX1 from LOOP+ node and thus remedy the stability margin reduction. If RX1 = 0, CX1

cannot exceed 10 nF. RX1 = 200Ω is recommended if it can be tolerated. Minimum RX1 = 40Ω if CX1 exceeds 10

nF.

CX3 also adversely affects stability of the loop and it must be limited to 20 pF. CX4 affects the control loop in the

same way as CX1, and it should be treated in the same way as CX1. CX2 is the only capacitance that improves

stability margins of the control loop. Its maximum size is limited only by the safety requirements.

Stability is a function of ILOOP as well. Since ILOOP is approximately equal to the collector current of the external

BJT, Gm of the BJT, and thus loop dynamics, depend on ILOOP. This dependence can be reduced by

degenerating the emitter of the BJT with a small resistance as discussed in Loop Interface. Inductance in series

with the LOOP+ and LOOP

− do not significantly affect the control loop.

Noise and Ripple

The output of the DAC is a current pulse train. The transition density varies throughout the DAC input code range

(ILOOP range). At the extremes of the code range, the transition density is the lowest which results in low

frequency components of the DAC output passing through the RC filter. Hence, the magnitude of the ripple

present in ILOOP is the highest at the ends of the transfer characteristic of the device (see TYPICAL

PERFORMANCE CHARACTERISTICS).

It should be noted that at wide noise measurement bandwidth, it is the ripple due to the

∑Δ modulator that

dominates the noise performance of the device throughout the entire code range of the DAC. This results in the

“U” shaped noise characteristic as a function of output current. At narrow bandwidths, and particularly at mid-

scale output currents, it is the amplifier driving the external BJT that starts to dominate as a noise source.

Digital Feedthrough

Digital feedthrough is indiscernible from the ripple induced by the

∑Δ modulator.

HART Signal Injection

The HART specification requires minimum suppression of the sensor signal in the HART signal band (1-2 kHz) of

about 60 dB. The filter in Figure 14 below meets that requirement.

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