Analog Devices AD604 Car Amplifier User Manual

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AD604

Rev. F | Page 14 of 32

PREAMPLIFIER

The input capability of the following single-supply DSX (2.5 ± 2 V

for a +5 V supply) limits the maximum input voltage of the

preamplifier to ±400 mV for the 14 dB gain configuration or

±200 mV for the 20 dB gain configuration.

The preamplifier gain can be programmed to 14 dB or 20 dB by

either shorting the FBK1 node to PAO1 (14 dB) or by leaving

the FBK1 node open (20 dB). These two gain settings are very

accurate because they are set by the ratio of the on-chip resistors.

Any intermediate gain can be achieved by connecting the

appropriate resistor value between PAO1 and FBK1 according

to Equation 2 and Equation 3.

()

65||7

RRRR

EXT

OUT

== (2)

()

[]

()

6567

7656

RRGRR

RRRGR

EXT

++×−

×+−×

= (3)

Because the internal resistors have an absolute tolerance of ±20%,

the gain can be in error by as much as 0.33 dB when R

EXT

is 30 Ω,

where it is assumed that R

EXT

is exact.

Figure 38 shows how the preamplifier is set to gains of 14 dB,

17.5 dB, and 20 dB. The gain range of a single channel of the

AD604 is 0 dB to 48 dB when the preamplifier is set to 14 dB

(Figure 38a), 3.5 dB to 51.5 dB for a preamp gain of 17.5 dB

(Figure 38b), and 6 dB to 54 dB for the highest preamp gain of

20 dB (Figure 38c).

FBK1

PAO1

COM1

PAI1

FBK1

PAO1

COM1

PAI1

FBK1

PAO1

COM1

PAI1

8Ω

32Ω

40Ω

a. PREAMP GAIN = 14dB

b. PREAMP GAIN = 17.5dB

c. PREAMP GAIN = 20dB

R10

40Ω

32Ω

8Ω

40Ω

32Ω

8Ω

00540-038

Figure 38. Preamplifier Gain Programmability

For a preamplifier gain of 14 dB, the −3 dB small signal bandwidth

of the preamplifier is 130 MHz. When the gain is at its maximum

of 20 dB, the bandwidth is reduced by half to 65 MHz. Figure 39

shows the ac response for the three preamp gains shown in

Figure 38. Note that the gain for an R

EXT

of 40 Ω should be

17.5 dB, but the mismatch between the internal resistors and

the external resistor causes the actual gain for this particular

preamplifier to be 17.7 dB. The −3 dB small signal bandwidth of

one complete channel of the AD604 (preamplifier and DSX) is

40 MHz and is independent of gain.

To achieve optimum specifications, power and ground manage-

ment are critical to the AD604. Large dynamic currents result

because of the low resistances needed for the desired noise

performance. Most of the difficulty is with the very low gain

setting resistors of the preamplifier that allow for a total input

referred noise, including the DSX, as low as 0.8 nV/√Hz. The

consequently large dynamic currents have to be carefully

handled to maintain performance even at large signal levels.

GAIN (dB)

OPEN

SHORT

50Ω

40Ω

150Ω

8Ω 32Ω

40Ω

100M10M1M100k

FREQUENCY (Hz)

EXT

00540-039

Figure 39. AC Response for Preamplifier Gains of 14 dB, 17.5 dB, and 20 dB

The preamplifier uses a dual ±5 V supply to accommodate large

dynamic currents and a ground referenced input. The preamplifier

output is also ground referenced and requires a common-mode

level shift into the single-supply DSX. The two external coupling

capacitors (C1 and C2 in Figure 37) connected to the PAO1 and

+DSXx, and –DSXx, nodes and ground, respectively, perform

this function (see the AC Coupling section). In addition, they

eliminate any offset that would otherwise be introduced by the

preamplifier. It should be noted that an offset of 1 mV at the

input of the DSX is amplified by 34.4 dB (× 52.5) when the gain

control voltage is at its maximum; this equates to 52.5 mV at the

output. AC coupling is consequently required to keep the offset

from degrading the output signal range.

The gain-setting preamplifier feedback resistors are small

enough (8  and 32 Ω) that even an additional 1 Ω in the

ground connection at Pin COM1 (the input common-mode

reference) seriously degrades gain accuracy and noise performance.

This node is sensitive, and careful attention is necessary to

minimize the ground impedance. All connections to the COM1

node should be as short as possible.

The preamplifier, including the gain setting resistors, has a

noise performance of 0.71 nV/√Hz and 3 pA/√Hz. Note that a

significant portion of the total input referred voltage noise is

due to the feedback resistors. The equivalent noise resistance

presented by R5 and R6 in parallel is nominally 6.4 Ω, which

contributes 0.33 nV/√Hz to the total input referred voltage noise.

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Analog Devices AD604 Car Amplifier User Manual