R

IN

R

IN

C

IN

0.1

µF

R

T

100

Ω

0.1

µF

1000pF

1:1

R

S

100

Ω

OPA695

R

1

400

Ω

A

(18dB)

R

2

57.5

Ω

V

IN

INP

INM

CM

−5V

+5V

10

Ω

ADS5520

ADS5520

Table 5. Recommended Amplifiers to Drive the Input of the ADS5520

INPUT SIGNAL FREQUENCY

RECOMMENDED AMPLIFIER

TYPE OF AMPLIFIER

USE WITH TRANSFORMER?

DC to 20 MHz

THS4503

Differential In/Out Amp

No

DC to 50 MHz

OPA847

Operational Amp

Yes

DC to 100 MHz

THS4509

Differential In/Out Amp

No

OPA695

Operational Amp

Yes

10 MHz to 120 MHz

THS3201

Operational Amp

Yes

THS3202

Operational Amp

Yes

Over 100 MHz

THS9001

RF Gain Block

Yes

When using single-ended operational amplifiers (such as the THS3201, THS3202, OPA695, or OPA847) 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 ADS5520. 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 ADS5520 directly, as shown in Figure 40, or with the addition of the filter circuit shown in Figure 41.

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 ADS5520 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 ADS5520 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 42), such amplifiers can be used for single-ended-to-differential conversion signal amplification.

Figure 41. Converting a Single-Ended Input Signal to a Differential Signal Using an RF Transformer

24

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