DAC08ESZ-REEL(RevC) Ver la hoja de datos (PDF) - Analog Devices

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DAC08ESZ-REEL
(Rev.:RevC)

8-Bit, High Speed, Multiplying D/A Converter (Universal Digital Logic Interface)

Analog Devices 

APPLICATION INFORMATION

REFERENCE AMPLIFIER SETUP

The DAC08 is a multiplying D/A converter in which the output

current is the product of a digital number and the input

reference current. The reference current may be fixed or may

vary from nearly zero to 4.0 mA. The full-scale output current

is a linear function of the reference current and is given by

I FR

=

255

256

×

I REF

where IREF = I14

In positive reference applications, an external positive reference

voltage forces current through R14 into the VREF(+) terminal

(Pin 14) of the reference amplifier. Alternatively, a negative

reference may be applied to VREF(–) at Pin 15; reference current

flows from ground through R14 into VREF(+) as in the positive

reference case. This negative reference connection has the

advantage of a very high impedance presented at Pin 15. The

voltage at Pin 14 is equal to and tracks the voltage at Pin 15 due

to the high gain of the internal reference amplifier. R15 (nomi-

nally equal to R14) is used to cancel bias current errors; R15

may be eliminated with only a minor increase in error.

Bipolar references may be accommodated by offsetting VREF or

Pin 15. The negative common-mode range of the reference

amplifier is given by VCM – = V− plus (IREF × 1 kΩ) plus 2.5 V.

The positive common-mode range is V+ less 1.5 V.

When a dc reference is used, a reference bypass capacitor is

recommended. A 5.0 V TTL logic supply is not recommended

as a reference. If a regulated power supply is used as a reference,

R14 should be split into two resistors with the junction bypas-

sed to ground with a 0.1 µF capacitor.

For most applications, the tight relationship between IREF and IFS

eliminates the need for trimming IREF. If required, full-scale

trimming can be accomplished by adjusting the value of R14, or

by using a potentiometer for R14. An improved method of full-

scale trimming that eliminates potentiometer T.C. effects is

shown in the recommended full-scale adjustment circuit

(Figure 27).

Using lower values of reference current reduces negative power

supply current and increases reference amplifier negative

common-mode range. The recommended range for operation

with a dc reference current is 0.2 mA to 4.0 mA.

DAC08

REFERENCE AMPLIFIER COMPENSATION FOR

MULTIPLYING APPLICATIONS

AC reference applications require the reference amplifier to be

compensated using a capacitor from Pin 16 to V−. The value of

this capacitor depends on the impedance presented to Pin 14;

for R14 values of 1.0 kΩ, 2.5 kΩ, and 5.0 kΩ, minimum values

of CC are 15 pF, 37 pF, and 75 pF. Larger values of R14 require

proportionately increased values of CC for proper phase margin,

so the ratio of CC (pF) to R14 (kΩ) = 15.

For fastest response to a pulse, low values of R14 enabling small

CC values should be used. If Pin 14 is driven by a high impedance

such as a transistor current source, none of the preceding values

suffice, and the amplifier must be heavily compensated, which

decreases overall bandwidth and slew rate. For R14 = 1 kΩ and

CC = 15 pF, the reference amplifier slews at 4 mA/µs, enabling a

transition from IREF = 0 to IREF = 2 mA in 500 ns.

Operation with pulse inputs to the reference amplifier can be

accommodated by an alternate compensation scheme. This

technique provides lowest full-scale transition times. An internal

clamp allows quick recovery of the reference amplifier from a

cutoff (IREF = 0) condition. Full-scale transition (0 mA to 2 mA)

occurs in 120 ns when the equivalent impedance at Pin 14 is

200 Ω and CC = 0. This yields a reference slew rate of 16 mA/µs,

which is relatively independent of the RIN and VIN values.

LOGIC INPUTS

The DAC08 design incorporates a unique logic input circuit

that enables direct interface to all popular logic families and

provides maximum noise immunity. This feature is made

possible by the large input swing capability, 2 µA logic input

current, and completely adjustable logic threshold voltage. For

V− = −15 V, the logic inputs may swing between −10 V and

+18 V. This enables direct interface with 15 V CMOS logic, even

when the DAC08 is powered from a 5 V supply. Minimum

input logic swing and minimum logic threshold voltage are

given by

V− + (IREF × 1 kΩ) + 2.5 V

The logic threshold may be adjusted over a wide range by placing

an appropriate voltage at the logic threshold control pin (Pin 1,

VLC). Figure 16 shows the relationship between VLC and VTH

over the temperature range, with VTH nominally 1.4 above VLC.

For TTL and DTL interface, simply ground Pin 1. When

interfacing ECL, an IREF = 1 mA is recommended. For interfacing

other logic families, see Figure 32. For general set-up of the

logic control circuit, note that Pin 1 sources 100 µA typical;

external circuitry should be designed to accommodate this

current.

Rev. C | Page 13 of 20

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