AD8335 datasheet(17/24 Pages) AD

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AD8335

Rev. 0 | Page 17 of 24

PREAMP

Although the preamp signal path is fully differential, the design

is optimized for single-ended input drive and signal source

resistance matching. Thus, the negative input to the differential

preamplifier Pins PMDx must be ac-grounded to provide a

balanced differential signal at the PrA outputs. Detailed

information regarding the preamplifier architecture is found in

the LNA section of the AD8331/AD8332 data sheet.

The preamplifier consists of a fixed gain amplifier with differen-

tial outputs. With the negative output available and a fixed gain

of 8 (18.06 dB), an active input termination is synthesized by

connecting a feedback resistor between the negative output and

the positive input, Pin PIPx. This technique is well known and

results in the input resistance shown in Equation 2.

)

(

(2)

where A/2 is the single-ended gain, or the gain from the PIPx

inputs to the PONx outputs. Since the amplifier has a gain of ×8

from its input to its differential output, it is important to note

that the gain A/2 is the gain from Pin PIPx to Pin PONx, which

is 6 dB lower, or 12.04 dB (×4). The input resistance is reduced

by an internal bias resistor of 14.7 kΩ in parallel with the source

resistance connected to Pin PIPx, with Pin PMDx ac-grounded.

Equation 3 can be used to calculate the needed RFB for a desired

RIN, and is used for higher values of RIN.

kΩ

)

( +

(3)

For example, to set RIN = 200 Ω, the value of RFB is 1.013 kΩ. If the

simplified Equation 2 is used to calculate RIN, the value is 197 Ω,

resulting in a less than 0.1 dB gain error. Factors such as a

widely varying source resistance might influence the absolute

gain accuracy more significantly. At higher frequencies, the

input capacitance of the PrA needs to be considered. The user

must determine the level of matching accuracy and adjust RFB

accordingly.

The bandwidths (BW) of the preamplifier and VGA are

approximately 110 MHz each, resulting in a cascaded BW of

approximately 80 MHz. Ultimately the BW of the PrA limits the

accuracy of the synthesized RIN. For RIN = RS up to approximately

200 Ω, the best match is between 100 kHz and 10 MHz, where

the lower frequency limit is determined by the size of the ac-

coupling capacitors, and the upper limit is determined by the

preamplifier BW. Furthermore, the input capacitance and RS

limits the BW at higher frequencies.

FREQUENCY (Hz)

100

100k

10M

50M

RIN = 500Ω, RFB = 2.5kΩ

RSH = ∞, CSH = 0pF

RIN = 200Ω, RFB = 1kΩ

RSH = 50Ω, CSH = 22pF

RIN = 100Ω, RFB = 499Ω

RIN = 50Ω, RFB = 249Ω

RSH = ∞, CSH = 0pF

RSH = 50Ω, CSH = 22pF

Figure 55. RIN vs. Frequency for Various Values of RFB.

Effects of RSH and CSH are also shown.

Figure 55 shows RIN vs. frequency for various values of RFB. Note

that at the lowest value, 50 Ω, RIN peaks at frequencies greater

than 10 MHz. This is due to the BW roll-off of the PrA as

mentioned earlier. The RSH and CSH network shown in Figure 58

reduces this peaking.

However, as can be seen for larger RIN values, parasitic

capacitance starts rolling off the signal BW before the PrA can

produce peaking and the RSH/CSH network further degrades the

match. Therefore RSH and CSH should not be used for values of

RIN greater than 50 Ω.

Noise

The total input referred noise (IRN) is approximately 1.3

nV/√Hz. Allowing for a gain of ×8 in the preamp, the VGA

noise is 0.46 nV/√Hz referred to the PrA input. The preamp

noise is 1.2 nV/√Hz. It is important to note that these noise

values include all amplifier noise sources, including the VGA

and the preamplifier gain resistors. Frequently, manufacturer

noise specifications exclude gain setting resistors, and the

voltage noise spectral density of an op amp might be presented

as 1 nV/√Hz. Including the gain resistors results in a much

higher noise specification.

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