ADS5500MPAPREP datasheet(25/37 Pages) TI1

25 / 37 page

ADS5500-EP

SGLS286C − JUNE 2005 – REVISED SEPTEMBER 2008

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This differential input topology produces a high level of

ac performance for high sampling rates. It also results

in a high usable input bandwidth, especially important

for high intermediate frequency (IF) or undersampling

applications. The ADS5500 requires each of the analog

inputs (INP, INM) to be externally biased around the

common-mode level of the internal circuitry (CM, pin

17). For a full-scale differential input, each of the

differential lines of the input signal (pins 19 and 20)

swings symmetrically between CM + 0.575 V and CM

– 0.575 V. This means that each input is driven with a

signal of up to CM

± 0.575 V, so that each input has a

maximum differential signal of 1.15 VPP for a total

differential input signal swing of 2.3 VPP. The maximum

swing is determined by the two reference voltages − the

top reference (REFP, pin 29), and the bottom reference

(REFM, pin 30).

The ADS5500 obtains optimum performance when the

analog inputs are driven differentially. The circuit shown

in Figure 6 shows one possible configuration using an

RF transformer.

1:1

INP

ADS5500

INM

ADT1−1WT

1nF

0.1

AC Signal

Source

Figure 6. Transformer Input to Convert

Single-Ended Signal to Differential Signal

The single-ended signal is fed to the primary winding of

an RF transformer. Since the input signal must be

biased around the common-mode voltage of the

internal circuitry, the common-mode voltage (VCM) from

the ADS5500 is connected to the center tap of the

secondary winding. To ensure a steady low-noise VCM

reference, best performance is obtained when the CM

(pin 17) output is filtered to ground with 0.1−

µF and

0.01-

µF low-inductance capacitors.

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

ADC input. When providing a custom CM level, be

aware that the input structure of the ADC sinks a

common-mode current in the order of 4 mA (2 mA per

input). Equation 1 describes the dependency of the

common-mode current and the sampling frequency:

4mA

125MSPS

Where: fS > 60 MSPS.

This equation designs the output capability and

impedance of the driving circuit accordingly.

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

incoming analog signal, it is possible to combine

single-ended operational

amplifiers

with

transformer or to use a differential input/output amplifier

without a transformer to drive the input of the ADS5500.

Texas

Instruments

offers

wide

selection

single-ended operational amplifiers (including the

THS3201, THS3202, OPA847, and OPA695) that can

be selected, depending on the application. An RF gain

block amplifier, such as the TI 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 minimize

even-order harmonics. For high 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 input of the

ADS5500 directly (see Figure 6) or with the addition of

the filter circuit (see Figure 7).

Figure 7 shows how RIN and CIN can be placed to isolate

the signal source from the switching inputs of the ADC

and to implement a low-pass RC filter to limit the input

noise in the ADC. It is recommended that these

components be included in the ADS5500 circuit layout

when any of the amplifier circuits discussed previously

are used. The components allow fine tuning of the

circuit performance. Any mismatch between the

differential lines of the ADS5500 input produces a

degradation in performance at high input frequencies,

mainly characterized by an increase in the even-order

harmonics. In this case, special care should be taken to

keep as much electrical symmetry as possible between

both inputs.

Another possible configuration for lower-frequency

signals is the use of differential input/output amplifiers

that can simplify the driver circuit for applications

requiring dc coupling of the input. Flexible in their

configurations (see Figure 8), such amplifiers can be

used for single-ended-to-differential conversion signal

amplification.

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