AFE8405 datasheet(27/152 Pages) TI

27 / 152 page

S0177-05

100 W

–

OPA695

imm

imp

400 W

0.1 F

AFE8405

0.1 F

1:1

–5V

57.5 W

AV=8V/V

(18dB)

100 W

1000pF

10 W

S0178-02

VOCM

inm

3.3V

10 F

0.1 F

THS4503

AFE8405

14-Bit/85-MSPS

1 F

10 F

0.1 F

–5V

10 W

0.1 F

inp

AFE8405

14-BIT, 85-MSPS, SINGLE-ADC, 8-CHANNEL WIDEBAND RECEIVER

www.ti.com

SLWS212 – OCTOBER 2008

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

ADC. 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 ADC directly, as shown in Figure 6-3, or with the addition of the

filter circuit shown in Figure 6-4.

Figure 6-4 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 AFE8405 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 ADC 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 6-5), such amplifiers can be used for single-ended-to-differential conversion

and signal amplification.

Figure 6-4. Converting a Single-Ended Input Signal to a Differential Signal Using an RF Transformer

Figure 6-5. Using the THS4503 With the AFE8405

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