Philips Semiconductors Linear Products

Product specification

AM6012

12-Bit multiplying D/A converter

August 31, 1994

781

testing and application. The logic switch design enables propagation

delays of only 25ns for each of the 12 bits. Settling time to within

LSB of the LSB is therefore 25ns, with each progressively larger bit

taking successively longer. The MSB settles in 250ns, thus

determining the overall settling time of 250ns. Settling to 10-bit

accuracy requires about 90 to 130ns. The output capacitance of the

AM6012 including the package is approximately 20pF; therefore, the

Settling time and propagation delay are relatively insensitive to logic

input amplitude and rise and fall times, due to the high gain of the

logic switches. Settling time also remains essentially constant for

values lies in the ability to attain a given output level with lower load

resistors, thus reducing the output RC time constant.

Measurement of settling time requires the ability to accurately

resolve

±2µA, therefore a 2.5kΩ load is needed to provide adequate

excessive RC damping of the output is difficult to prevent while

maintaining adequate sensitivity. However, the major carry from

011111111111 to 100000000000 provides an accurate indicator of

settling time. This code change does not require the normal 6.2 time

constants to settle to within

±0.1% of the final value, and thus

AM6012 switching transients or “glitches” are very low and may be

further reduced by small capacitive loads at the output at a minor

sacrifice in settling time.

Fastest operation can be obtained by using short leads, minimizing

output capacitance and load resistor values, and by adequate

not require large electrolytic bypass capacitors as the supply current

drain is independent of input logic states; 0.1

µF capacitors at the

supply pins provide full transient protection.

APPLICATIONS INFORMATION

Reference Amplifier Setup

The AM6012 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 +1.0mA. The full range output current is a linear function of

the reference current and is given by:

I

4095

4096

x 4 x (I

In positive reference applications, an external positive reference

of the reference amplifier. Alternatively, a negative reference may be

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

bias current errors (Figure 2a).

15. The negative common-mode range of the reference amplifier is

common-mode range is V+ less 1.23V.

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

recommended. A 5.0V TTL logic supply is not recommended as a

should be split into two resistors with the junction bypassed to

ground with a 0.1

µF capacitor.

MULTIPLYING OPERATION

The AM6012 provides excellent multiplying performance with an

1mA to 1

µA. Monotonic operation is maintained over a typical range

REFERENCE AMPLIFIER COMPENSATION FOR

MULTIPLYING APPLICATIONS

reference applications will 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, 2.5 and 5.0k

25pF. Larger values of R14 require proportionately increased values

of CC for proper phase margin (see Figure 2b).

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

as a transistor current source, none of the above values will suffice

and the amplifier must be heavily compensated which will decrease

reference amplifier slews at 4mA/ms enabling a transition from

Operation with pulse inputs to the reference amplifier may 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

when the equivalent impedance at Pin 14 is 800

yields a reference slew rate of 8mA/

µs which is relatively

LOGIC INPUTS

The AM6012 design incorporates a unique logic input circuit which

enables direct interface to all popular logic families and provides

maximum noise immunity. This feature is made possible by the large

input swing capability, 40

µA logic input current, and completely

adjustable logic threshold voltage. For V-=-15V, the logic inputs may

swing between -5 and +10V. This enables direct interface with +15V

CMOS logic, even when the AM6012 is powered from a +5V supply.

Minimum input logic swing and minimum logic threshold voltage are

given by:

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

an appropriate voltage at the logic threshold control pin (Pin 13,

control circuit, it should be noted that Pin 13 will sink 1.1mA typical.

External circuitry should be designed to accommodate this current

(Figure 3).