AD9671EBZ datasheet(23/61 Pages) AD

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AD9671

Data Sheet

Rev. A| Page 22 of 60

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 the 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 feed-

back 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 (RIN) calculation is shown in Equation 4.

)

(

)

(

)

(

FB1

















(4)

where:

RFB1 and RFB2 are the external feedback resistors.

20 Ω is the internal switch on resistance.

30 Ω is an internal series resistance common to the two internal

switches.

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

the LO-x outputs.

RFB can be 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 Pin LI-x to Pin LO-x 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 Pin LI-x, with

Pin LG-x ac grounded. Use the more accurate Equation 5 to

calculate the required RFB for a desired RIN, even for higher

values of RIN.



















)

(

)

(

)

(

FB1

(5)

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

12.1 dB (4×), the value of RFB1 from Equation 1 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). Use Register 0x02C in the SPI memory to

program the AD9671 for four impedance matching options:

three active terminations and unterminated. Table 8 shows an

example of how to select RFB1 and RFB2 for 66 Ω, 100 Ω, and 200 Ω

input impedance for LNA gain = 21.6 dB (12×).

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

RFB1 = 650 Ω, RFB2 = 1350 Ω

Addr. 0x02C

Value

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

∞

1 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. For RIN = RS up to about 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 RIN vs. frequency for various values of RFB.

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. RIN vs. Frequency for Various Values of RFB

(Effects of RSH and CSH Are Also Shown)

However, 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.

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