ADS5553IPFPR datasheet(17/24 Pages) TI

17 / 24 page

ADS5553

SLWS158 − FEBRUARY 2005

www.ti.com

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 ADS5553 requires each of the analog

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

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

1 and 20). For a full-scale differential input, each of the

differential lines of the input signal 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 (REFPA, pin 7

and REFPB, pin 15) and the bottom reference (REFMA,

pin 6 and REFMB, pin 18).

The ADS5553 obtains optimum performance when the

analog inputs are driven differentially. The circuit shown

in Figure 29 shows one possible configuration using an

RF transformer.

0.1 µF

Signal

Source

50 Ω

10 Ω

INP

INM

ADS5553

25 Ω

ADT1−1WT

1:1

50 Ω

100 nF

25 Ω

100 nF

25 Ω

Figure 29. 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 ADS5553 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

output (pins 1 and 20) is filtered to ground with a 10 Ω

series resistor and parallel 0.1 µF and 0.001 µF

low-inductance capacitors as shown in Figure 29.

Output VCM (pins 1 and 20) 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 200 µA (100 µA

per input). Equation (1) describes the dependency of

the common-mode current and the sampling

frequency:

20)

400 mA

fs(in MSPS)

125MSPS

This equation helps to design 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 an RF

transformer, or to use a differential input/output

amplifier without a transformer, to drive the input of the

ADS5553. Texas Instruments offers a wide selection of

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 Texas Instruments THS9001,

can also be used with an RF transformer for high input

frequency

applications.

The

THS4503/6

are

recommended differential input/output amplifiers.

Table 1 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

ADS5553. 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

ADS5553 directly, as shown in Figure 29 or with the

addition of the filter circuit shown in Figure 30.

Figure 30 illustrates 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 ADS5553 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 ADS5553 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

(1)

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