ADC

ADC

ADC

ADC

ADC

AS1419

AS1419A

AS1419 & AS1419A

Rev. 1.5 08/09

Austin Semiconductor, Inc. reserves the right to change products or specifications without notice.

11

Austin Semiconductor, Inc.

100

Ω source resistor to limit the input bandwidth to 1.6MHz.

The 1000pF capacitor also acts as a charge reservoir for the

input sample-and-hold and isolates the ADC input from

sampling glitch sensitive circuitry. High quality capacitors and

resistors should be used since these components can add

distortion. NPO and silver mica type dielectric capacitors have

excellent linearity. Carbon surface mount resistors can also

generate distortion from self heating and from damage that may

occur during soldering. Metal film surface mount resistors are

much less susceptible to both problems.

Input Range

The ±2.5V input range of the AS1419 is optimized for low

noise and low distortion. Most op amps also perform well over

this same range, allowing direct coupling to the analog inputs

and eliminating the need for special translation circuitry.

Some applications may require other input ranges. The

AS1419 differential inputs and reference circuitry can

accommodate other input ranges often with little or no

additional circuitry. The following sections describe the

reference and input circuitry and how they affect the input

range.

Internal Reference

The AS1419 has an on-chip, temperature compensated,

curvature corrected, bandgap reference that is factory trimmed

to 2.500V. It is connected internally to a reference amplifier and

is available at V

REF

(Pin 3) see Figure 8a. A 2k resistor is in series

with the output so that it can be easily overdriven by an exter-

nal reference or other circuitry, see Figure 8b. The reference

amplifier gains the voltage at the V

REF

pin by 1.625 to create the

required internal reference voltage. This provides buffering

between the V

REF

pin and the high speed capacitive DAC. The

reference amplifier compensation pin (REFCOMP, Pin 4) must

be bypassed with a capacitor to ground. The reference

amplifier is stable with capacitors of 1µF or greater. For the best

noise performance, a 10µF ceramic or 10µF tantalum in parallel

with a 0.1µF ceramic is recommended.

The V

REF

pin can be driven with a DAC or other means

shown in Figure 9. This is useful in applications where the peak

input signal amplitude may vary. The input span of the ADC

can then be adjusted to match the peak input signal, maximizing

the signal-to-noise ratio. The filtering of the internal AS1419

reference amplifier will limit the bandwidth and settling time of

this circuit. A settling time of 5ms should be allowed for after a

reference adjustment.