LT Mag电路精选V：1996-1998年间的数据转换与接口电路

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"Application Note 87是线性技术杂志系列文章的第五部分，旨在从该杂志的第六卷第一期（1996年2月）到第八卷第四期（1998年11月）中选取有价值的数据转换、接口和信号调理电路，以便长期保存这些技术精华。与前作AN67类似，AN87涵盖了高速视频、接口和热插拔电路设计、主动RC滤波器和开关电容电路，以及多种数据转换和测量电路。此外，还有一些无法简单归类的独特设计，作者们将在文中详细介绍各自的电路。这篇应用笔记的作者是Richard Markell，他担任了《线性技术杂志》电路收藏系列的编辑。Note 87在2000年11月发布，其内容丰富多样，不仅关注传统的数据转换技术，还涵盖了如何处理现代电子系统中的高速数据传输、精确信号处理以及设备间的高效通信。对于那些对数据采集、信号处理和接口设计感兴趣的工程师而言，AN87提供了宝贵的实践经验和参考案例。值得注意的是，由于文章标题完全保留了《线性技术杂志》中的原文风格，可能会导致术语使用的不一致性，但这种保持原貌的做法确保了读者能直接接触到原始文献中的专业语言和设计理念。通过阅读AN87，读者不仅可以学习到最新的电路设计技术，还能了解到当时技术发展的一个重要历史阶段。《线性技术杂志电路收藏》第五卷的Application Note 87是一份深入且实用的资源，对于提升电子工程技能和理解行业发展趋势具有重要意义。"

Application Note 87

AN87-16

–

DATA IN

SERIAL CLOCK

CHIP SELECT/

DAC LOAD

DATA OUT

CLEAR

CLK

CS/LD

OUT

CLR

24-BIT SHIFT REGISTER AND LATCH

DAC

0.1µF

IN A

IN B

REF B

FB B

REF A

FB A

OUT1

OUT2

OUT1

U3A

1/2 LT1358

U3B

1/2 LT1358

15V

–15V

0.01µF

15V

0.01µF

OUT A

OUT B

U1 LTC1590

DI1590_03.EPS

AGND

DGND

–

U2B

1/2 LT1358

–

U4B

1/2 LT1358

–

U4A

1/2 LT1358

–15V

0.01µF

–

U2A

1/2 LT1358

15V

0.01µF

10k

n+1

• π • R

• C

Figure 15. This LTC1590-Controlled Dual Single-Pole Lowpass Filter Uses R

and the DAC’s Input Code to Create

an Effective Resistance that Sets the Integrator’s Time Constant and, Therefore, the Circuit’s Cutoff Frequency.

NEW 16-BIT SO-8 DAC HAS 1LSB MAX INL AND DNL

OVER

INDUSTRIAL

TEMPERATURE

by Jim Brubaker and William C. Rempfer

New generations of industrial systems are moving to 16

bits and hence require high performance 16-bit data

converters. The new LTC1595/LTC1596 16-bit DACs pro-

vide the easiest to use, most cost effective, highest perfor-

mance solution for industrial and instrumentation applica-

tions. The LTC1595/LTC1596 are serial input, 16-bit,

multiplying current output DACs. Features of the new

DACs include:

❏ ±1LSB maximum INL and DNL over the industrial

temperature range

❏ Ultralow, 1nV-s glitch impulse

❏ ±10V output capability

❏ Small SO-8 package (LTC1595)

❏ Pin-compatible upgrade for industry-standard 12-bit

DACs (DAC8043/8143 and AD7543)

Application Note 87

AN87-17

0V–10V and ±10V Output Capability

Precision 0V–10V Outputs with One Op Amp

Figure 16 shows the circuit for a 0V–10V output range.

The DAC uses an external reference and a single op amp

in this conﬁguration. This circuit can also perform 2-

quadrant multiplication where the reference input is driven

by a ±10V input signal and V

OUT

swings from 0V to –V

REF

The full-scale accuracy of the circuit is very precise

because it is determined by precision-trimmed internal

resistors. The power dissipation of the circuit is set by the

op amp dissipation and the current drawn from the DAC

reference input (7k nominal). The supply current of the

DAC itself is less than 10µA.

An advantage of the LTC1595/LTC1596 is the ability to

choose the output op amp to optimize the accuracy,

speed, power and cost of the application. Using an LT1001

provides excellent DC precision, low noise and low power

dissipation (90mW total for Figure 16’s circuit). For higher

speed, an LT1007, LT1468 or LT1122 can be used. The

LT1122 will provide settling to 1LSB in 3µs for a full-scale

transition. Figure 17 shows the 3µs settling performance

obtained with the LT1122. The feedback capacitor in

Figure 16 ensures stability. In higher speed applications,

it can be used to optimize transient response. In slower

applications, the capacitor can be increased to reduce

glitch energy and provide ﬁltering.

REF

LTC1595

GND

REF

(–10V TO 10V)

CLOCK

DATA

LOAD

CLK

SRI

OUT1

33pF

OUT

0V TO

–V

REF

1595_04.EPS

–

LT1001

µP

0.1µF

REF

LTC1595

GND

REF

(–10V TO 10V)

CLK

SRI

OUT1

OUT

(–V

REF

REF)

1595_05.EPS

–

1/2 LT1112

20k

10k

20k

–

1/2 LT1112

33pF

Figure 16. With a Single External Op Amp, the DAC

Performs 2-Quadrant Multiplication with ±10V Input and

0V to –V

REF

Output. With a Fixed –10V Reference, it

Provides a Precision 0V–10V Unipolar Output.

Figure 17. When Used with an LT1122 (in the Circuit of Figure

16), the LTC1595/LTC1596 Can Settle in 3µs to a Full-Scale

Step. The Top Trace Shows the Output Swinging from 0V to 10V.

The Bottom Trace Shows the Gated Settling Waveform Settling

to 1LSB (1/3 of a Division) in 3µs.

OUT

5V/DIV

GATED V

OUT

500µV/DIV

1µs/DIV

Figure 18. With a Dual Op Amp, the DAC Performs 4-Quadrant Multiplication.

With a Fixed 10V Reference, it Provides a ±10V Bipolar Output.

Application Note 87

AN87-18

LTC1659, LTC1448: SMALLEST RAIL-TO-RAIL

12-BIT DACS HAVE LOWEST POWER

by Hassan Malik

In this age of portable electronics, power and size are the

primary concerns of most designers. The LTC1659 and

the LTC1448 are rail-to-rail, 12-bit, voltage output DACs

that address both of these concerns. The LTC1659 is a

single DAC in an MSOP-8 package that draws only 250µA

from a 3V or 5V supply, whereas the LTC1448 is a dual

DAC in an SO-8 package that draws 450µA from a 3V or 5V

supply.

Figure 19 shows a convenient way to use the LTC1659 in

a digital control loop where 12-bit resolution is required.

The output of the LTC1659 will swing from 0V to V

REF

because there is a gain of one from the REF pin to V

OUT

full-scale. Because the output can only swing up to V

REF

should be less than or equal to V

to prevent the loss

of codes and degradation of PSRR near full-scale.

To obtain full dynamic range, the REF pin can be connected

to the supply pin, which can be driven from a reference to

guarantee absolute accuracy (see Figure 20). The LT1236

is a precision 5V reference with an input range of 7.2V to

40V. In this conﬁguration, the LTC1659 has a wide output

swing of 0V to 5V. The LTC1448 can be used in a similar

configuration where dual DACs are needed.

Precision ±10V Outputs with a Dual Op Amp

Figure 18 shows a bipolar, 4-quadrant multiplying appli-

cation. The reference input can vary from –10V to 10V and

OUT

swings from –V

REF

to +V

REF

. If a ﬁxed 10V reference

is used, a precision ±10V bipolar output will result.

Unlike the unipolar circuit of Figure 16, the bipolar gain

and offset will depend on the matching of the external

µP

1659_01

2.7V to 5.5V

CONTROL

VOLTAGE

(0V TO V

REF

)

REF

≤ V

REF

CLK

OUT

LTC1659

CS/LD GND

µP

CONTROL

VOLTAGE

(OV TO 5V)

(7.2V to 40V)

REF

CLK

OUT

LTC1659

CS/LD GND

1659_02

IN OUT

GND

LT1236

0.1µF

resistors. A good way to provide good matching and save

board space is to use a pack of matched 20k resistors (the

10k unit is formed by placing two 20k resistors in parallel).

The LT1112 dual op amp is an excellent choice for high

precision, low power applications that do not require high

speed. The LT1469 or LT1124 will provide faster settling.

Again, with op amp selection the user can optimize the

speed, power, accuracy and cost of the application.

Figure 19. 12-Bit DAC for Digital Control Loop

Figure 20. 12-Bit DAC with Wide Output Swing

Application Note 87

AN87-20

Interface Circuits

SIMPLE RESISTIVE SURGE PROTECTION FOR

INTERFACE CIRCUITS

by LTC Applications Staff

Surges and Circuits

Many interface circuits must survive surge voltages such

as those created by lightning strikes. These high voltages

cause the devices within the IC to break down and conduct

large currents, causing irreversible damage to the IC.

Engineers must design circuits that tolerate the surges

expected in their environments. They can quantify the

surge tolerance of circuitry by using a surge standard.

Standards differ mainly in their voltage levels and wave

forms. At LTC, we test surge resistance using the circuit

of Figure 22. We describe the voltage wave form (Figure

23) by its peak value V

, the “front time” T

(roughly, the

rise time), and the “time to half-value” T

1/2

(roughly, the

time from the beginning of the pulse to when the pulse

decays to half of V

). Surges are similar to ESD, but

challenge circuits in a different way. A surge may rise to

1kV in 10ms, whereas an ESD pulse might rise to 15kV in

only a few ns. However, the surge lasts for more than

100ms, whereas the ESD pulse decays in about 50ns.

Thus, the surge challenges the power dissipation ability of

the protection circuitry, whereas the ESD challenges the

turn-on time and peak current handling. The Linear Tech-

nology LT1137A has on-chip circuitry to withstand ESD

pulses up to 15kV (IEC 801-2). This circuitry also in-

creases the surge tolerance of the LT1137A relative to a

standard 1488/1489.

DUT

OUT

0.05µF

4kV

7µF

3kV

HV SUPPLY

50Ω

75Ω, 2W

10k

CONTROLLED BY R2

OUT

1/2

CONTROLLED BY C1

SET BY HV SUPPLY

TIME

1/2

CONTROLLED BY R2

OUT

1/2

CONTROLLED BY C1

SET BY HV SUPPLY

VOLTAGE

~ 10µs

1/2

= 120µs

Designing for Surge Tolerance

Many designers enhance the surge tolerance of a circuit

by placing a transient voltage suppressor (TVS) in parallel

with the vulnerable IC pins, as shown in Figure 24. The

TVS contains Zener diodes, which break down at a certain

voltage and shunt the surge current to ground. Thus, the

TVS clamps the voltage at a level safe for the IC. The TVS,

like any protection circuitry, increases the manufacturing

cost and complexity of the circuit. Alternately, designers

can use a series resistor to protect the vulnerable pins, as

shown in Figure 25. The resistor reduces the current

0.1µF

–

1488

TVS

Figure 22. LTC Surge-Test Circuit: T

Controlled by R2 • C

OUT

; T

1/2

Controlled by C1 • R1; V

Set by HV Supply

Figure 23. LTC Surge-Test Waveform

Figure 24. 1488 Line Driver with TVS Surge Protection

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