Datasheet

LTC6430-15

643015f

applicaTions inForMaTion

Figure 5. ADC Output: 1-Tone Test at 380MHz with 310Msps Sampling Rate Undersampled in the Third Nyquist Zone

pads should be avoided within these distances.

A 100nH shunt inductor at the ADC input approximates

the complex conjugate of the ADC sampling circuit, and in

doing so, improves power transfer and suppresses the low

frequency difference products produced by direct sampling

ADCs. If the entire frequency range from 300MHz to 1GHz

were of interest, a 100nH inductor at the input is acceptable,

but if interest is only in higher frequencies, performance

would be better if the input inductor is reduced in value.

If lower frequencies are of interest, a higher value up to

some 200nH may be practical, but beyond that range the

SRF of the inductor becomes an issue. As this inductor

is placed at different distances either before or after the

ADC inputs, the optimal value may change. In all cases, it

should be within 50ps of the ADC inputs. End termination

may be more than 200ps distant if after the ADC. If the

end termination were perfect, it could be at any distance

after the ADC. To terminate the input path after the ADC,

place the termination resistors

on the back of the PCB. If

the input signal path is buried or on the back of the PCB,

termination resistors should be placed on the top of the

PCB to properly terminate after the ADC.

Although the ADC is isolated by a driver amplifier, care

must be taken when filtering at the amplifier input. Much

like MESFETs, high frequency mixing products are handled

well by the LTC6430. However, if there is no band limiting

after the LTC6430, these mixing products, reduced by

reverse isolation but subsequently reflected from a filter

prior to the LTC6430 and reamplified, can cause distor-

tion. In such cases, the network will then be sensitive to

transmission line lengths and impedance characteristics

of the filter prior to the LTC6430. Diplexers or absorptive

filters can produce more robust results. An absorptive

filter or diplexer-like structure after the amplifier reduces

the sensitivity to the network prior to the amplifier, but the

same constraints previously outlined apply to the filter.