ADS5500IPAP datasheet(23/37 Pages) TI2

23 / 37 page

www.ti.com

600 mA

125MSPS

(5)

ADS5500

SBAS303F – DECEMBER 2003 – REVISED FEBRUARY 2007

can be selected depending on the application. An RF

gain block amplifier, such as TI's THS9001, can also

be used with an RF transformer for very high input

frequency

applications.

The

THS4503

recommended

differential

input/output

amplifier.

Table 4 lists the recommended amplifiers.

When

using

single-ended

operational

amplifiers

(such as the THS3201, THS3202, OPA847, or

OPA695) to provide gain, a three-amplifier circuit is

recommended with one amplifier driving the primary

of an RF transformer and one amplifier in each of the

legs of the secondary driving the two differential

inputs of the ADS5500. These three amplifier circuits

Figure 42. Transformer Input to Convert

minimize

even-order

harmonics.

For

very

high

Single-Ended Signal to Differential Signal

frequency inputs, an RF gain block amplifier can be

used to drive a transformer primary; in this case, the

transformer secondary connections can drive the

The single-ended signal is fed to the primary winding

input

the

ADS5500

directly,

shown

of an RF transformer. Since the input signal must be

Figure 42, or with the addition of the filter circuit

biased around the common-mode voltage of the

shown in Figure 43.

internal circuitry, the common-mode voltage (VCM)

from the ADS5500 is connected to the center-tap of

Figure 43 illustrates how RIN and CIN can be placed

the secondary winding. To ensure a steady low-noise

to isolate the signal source from the switching inputs

VCM reference, best performance is obtained when

of the ADC and to implement a low-pass RC filter to

the CM (pin 17) output is filtered to ground with 0.1

limit the input noise in the ADC. It is recommended

µF and 0.001-µF low-inductance capacitors.

that these components be included in the ADS5500

circuit layout when any of the amplifier circuits

Output VCM (pin 17) is designed to directly drive the

discussed previously are used. The components

ADC input. When providing a custom CM level, be

allow fine-tuning of the circuit performance. Any

aware that the input structure of the ADC sinks a

mismatch between the differential lines of the

common-mode current in the order of 600

µA (300

ADS5500

input

produces

degradation

µA per input) at 125 Msps. Equation 5 describes the

performance

high

input

frequencies,

mainly

dependency of the common-mode current and the

characterized by an increase in the even-order

sampling frequency:

harmonics. In this case, special care should be taken

to keep as much electrical symmetry as possible

between both inputs.

Where:

Another possible configuration for lower-frequency

fS > 2 Msps.

signals

the

use

differential

input/output

amplifiers that can simplify the driver circuit for

This equation helps to design the output capability

applications

requiring

dc-coupling

the

input.

and impedance of the driving circuit accordingly.

Flexible in their configurations (see Figure 44), such

When it is necessary to buffer or apply a gain to the

amplifiers

can

used

for

single-ended-

incoming analog signal, it is possible to combine

to-differential conversion, signal amplification.

single-ended operational amplifiers with an RF

transformer, or to use a differential input/output

amplifier without a transformer, to drive the input of

the

ADS5500.

offers

wide

selection

single-ended operational amplifiers (including the

THS3201, THS3202, OPA847, and OPA695) that

Submit Documentation Feedback

ADS5500IPAP Datasheet(PDF) 23 Page - Texas Instruments

ADS5500IPAP Datasheet(HTML) 23 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

Datasheet Download

Link URL