AD9674KBCZ datasheet(22/48 Pages) AD

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Data Sheet

AD9674

Rev. A | Page 21 of 47

Low value feedback resistors and the current driving capability

of the output stage allow the LNA to achieve a low input referred

noise voltage of 0.78 nV/√Hz (at a gain of 21.6 dB). On-chip

resistor matching results in precise single-ended gains, which are

critical for accurate impedance control. The use of a fully

differential topology and negative feedback minimizes distortion.

Low second-order harmonic distortion is particularly important

in harmonic ultrasound imaging applications.

Active Impedance Matching

The LNA consists of a single-ended voltage gain amplifier with

differential outputs; the negative output is externally available on

two output pins (LO-x and LOSW-x) that are controlled via

internal switches. This configuration allows active input impedance

synthesis of three different impedance values (and an unterminated

value) by connecting up to two external resistances in parallel

and controlling the internal switch states via the SPI. For example,

with a fixed gain of 8× (17.9 dB), an active input termination is

synthesized by connecting a feedback resistor between the negative

output pin, LO-x, and the positive input pin, LI-x. This well-known

technique is used for interfacing multiple probe impedances to a

single system. The input resistance calculation is shown in

Equation 4.











 +

Ω

)

(

)

(

FB2

FB1

(4)

where A/2 is the single-ended gain or the gain from the LI-x

inputs to the LO-x outputs, RFB1 and RFB2 are the external feedback

resistors, the 20 Ω is the internal switch on resistance, and the 30 Ω

is an internal series resistance common to the two internal

switches. RFB can equal to RFB1, RFB2, or (RFB1+ 20 Ω)||(RFB2+ 20 Ω)

depending on the connection status of the internal switches.

Because 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 the LI-x pin to the LO-x pin, and that it is 6 dB

less than the gain of the amplifier, or 12.1 dB (4×). The input

resistance is reduced by an internal bias resistor of 6 kΩ in parallel

with the source resistance connected to the LI-x pin and with the

LG-x pin ac grounded. Equation 5 can be used to calculate the

required RFB for a desired RIN, even for higher values of RIN.

Ω











 +

Ω

)

(

)

(

FB2

FB1

(5)

For example, to set RIN to 200 Ω with a single-ended LNA gain of

12.1 dB (4×), the value of RFB from Equation 4 must be 950 Ω

while the switch for RFB2 is open. If the more accurate equation

(Equation 5) is used to calculate RIN, the value is then 194 Ω

instead of 200 Ω, resulting in a gain error of less than 0.27 dB.

Some factors, such as the presence of a dynamic source resistance,

may influence the absolute gain accuracy more significantly.

At higher frequencies, the input capacitance of the LNA must be

considered. The user must determine the level of matching

accuracy and adjust RFB accordingly.

RFB is the resulting impedance of the RFB1 and RFB2 combination (see

Figure 33). Using Address 0x02C in the SPI memory, the AD9674

can be programmed for four impedance matching options: three

active terminations and one unterminated option. Table 8 shows an

example of how to select RFB1 and RFB2 for RIN = 66 Ω, 100 Ω, and

200 Ω input impedances for an LNA gain = 21.6 dB (12×).

Table 8. Active Termination Example for LNA Gain = 21.6 dB,

RFB1 = 650 Ω, and RFB2 = 1350 Ω

Reg. 0x02C,

Bits[1:0]

RS (Ω)

LO-x

Switch

LOSW-x

Switch

RFB (Ω)

RIN (Ω)

(Eq. 4)

00 (default)

100

Off

RFB1

100

RFB1||RFB2

200

Off

RFB2

200

N/A1

Off

∞

N/A means not applicable.

The bandwidth (BW) of the LNA is greater than 80 MHz. Ultimately,

the BW of the LNA limits the accuracy of the synthesized RIN. 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 LNA BW. Furthermore, the input capacitance

and RS limit the BW at higher frequencies. Figure 34 shows input

resistance (RIN) vs. frequency for various RFB values.

100

100k

10M

100M

FREQUENCY (Hz)

RS = 50Ω, RFB = 200Ω, CSH = 70pF

RS = 100Ω, RFB = 400Ω, CSH = 20pF

RS = 200Ω, RFB = 800Ω

RS = 500Ω, RFB = 2kΩ

Figure 34. Input Resistance (RIN) vs. Frequency for Various RFB Values

(Effects of RS and CSH Are Also Shown)

For larger RIN values, parasitic capacitance starts rolling off the

signal BW before the LNA can produce peaking. CSH further

degrades the match; therefore, do not use CSH for values of RIN

that are greater than 100 Ω (see Figure 34).

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