DAC161S997RGHT datasheet(13/27 Pages) TI

13 / 27 page

DACCODE (hex)

24.0

21.5

20.0

4.0

3.5

0.4

0.2

Programmable IERROR

MIN(ILOOP) ± Loop Powered

MIN(ILOOP) ± Self Powered

DAC161S997

www.ti.com

SNAS621 – JUNE 2013

Figure 10 shows the variant of the transmitter where the local supply provides supply currents to the system

blocks, and not the 4-20mA loop Self-Powered Transmitter. The ame basic relationship between the ILOOP and

IDAC continues, but the component currents of ILOOP are only IDAC and IE.

DC Input-Output Transfer Function

The output current sourced by the OUT pin of the device is expressed by Equation 2.

ILOOP = 24 mA (DACCODE / 2

16)

(2)

The valid DACCODE range is the full 16-bit code space (0x0000 to 0xFFFF), resulting in the IDAC range of 0 to

approximately 12

μA, which, however, does not result in the ILOOP range of 0 to 24 mA. The maximum output

current sourced out of OUT pin, ILOOP, is 24 mA. The minimum output current is dependent on the system

implementation. The minimum output current is the sum of the supply currents of the DAC161S997 internal

blocks, IA, ID, and companion devices present in the system, IAUX. The last component current, IE, is theoretically

controlled down to 0, however, due to the stability considerations of the control loop, not allowing the IE to drop

below 200

μA is advised.

The graph in Figure 11 shows the DC transfer characteristic of the 4-20mA transmitter, including minimum

current limits. The minimum current limit for the Loop-Powered Transmitter is typically around 400

μA (ID+ IA +

IAUX + IE). The minimum current limit for the Self-Powered Transmitter is typically around 200 μA (IE). Typical

values for ID and IA are listed in the ELECTRICAL CHARACTERISTICS table. IE depends on the BJT device

used.

Figure 11. DAC-DC Transfer Function

Loop Interface

The DAC161S997 cannot directly interface to the typical 4 - 20 mA loop due to the excessive loop supply

voltage. The loop interface has to provide the means of stepping down the LOOP Supply to 3.6V. This can be

accomplished with either a linear regulator (LDO) or switching regulator while keeping in mind that the regulator’s

quiescent current will have direct effect on the minimum achievable ILOOP (see DC Input-Output Transfer

Function).

The second component of the loop interface is the external NPN transistor (BJT). This device is part of the

control circuit that regulates the transmitter’s output current (ILOOP). Since the BJT operates over the wide current

range, spanning at least 4 - 20 mA, it is necessary to degenerate the emitter in order to stabilize transistor’s

transconductance (gm). The degeneration resistor of 22Ω is suggested in typical applications. For circuit details,

see Figure 22.

The NPN BJT should not be replaced with an N-channel FET (Field Effect Transistor) for the following reasons:

discrete FET’s typically have high threshold voltages (VT), in the order of 1.5V to 2V, which is beyond the BASE

output maximum range; discrete FET’s present higher load capacitance which may degrade system stability

margins; and BASE output relies on the BJT’s base current for biasing.

Product Folder Links: DAC161S997

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