AD8336ACPZ-R7 datasheet(21/27 Pages) AD

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Data Sheet

AD8336

Rev. F | Page 21 of 27

The tap resistance of the resistors within the R/2R ladder is

640 Ω/3, or 213.3 Ω, and is the Johnson noise source of the

attenuator.

SETTING THE GAIN

The overall gain of the AD8336 is the sum (in decibels) or the

product (magnitude) of the preamplifier gain and the VGA gain.

The preamplifier gain is calculated as with any operational

amplifier, as seen in the Applications Information section. It is

most convenient to think of the device gain in exponential terms

(that is, in decibels) since the VGA responds linearly in decibels

with changes in control voltage VGAIN at the gain pins.

The gain equation for the VGA is

(V)

(dB)



















GAIN

Gain

VGA

where VGAIN = VGPOS − VGNEG.

The gain and gain range of the VGA are both fixed at 34 dB and

60 dB, respectively; thus, the composite device gain is changed by

adjusting the preamplifier gain. For a preamplifier gain of 12 dB

(4×), the composite gain is −14 dB to +46 dB. Therefore, the

calculation for the composite gain (in decibels) is

Composite Gain = GPRA + [VGAIN (V) × 49.9 dB/V] + 4.4 dB

For example, the midpoint gain when the preamplifier gain is

12 dB is

12 dB + [0 V × 49.9 dB/V] + 4.4 dB = 16.4 dB

Figure 3 is a plot of gain in decibels vs. VGAIN in millivolts, when

the preamplifier gain is 12 dB (4×). Note that the computed

result closely matches the plot of actual gain.

In Figure 3, the gain slope flattens at the limits of the VGAIN

input. The gain response is linear in dB over the center 80% of

the control range of the device. Figure 78 shows the ideal gain

characteristics for the VGA stage gain, the composite gain, and

the preamplifier gain.

VGAIN (V)

–10

FOR PREAMP GAIN = 26dB

–30

–20

FOR PREAMP GAIN = 6dB

GAIN CHARACTERISTICS

COMPOSITE GAIN

VGA STAGE GAIN

USABLE GAIN RANGE OF

AD8336

FOR PREAMP GAIN = 12dB

0.5

0.7

0.3

0.1

–0.1

–0.3

–0.5

–0.7

Figure 78. Ideal Gain Characteristics of the AD8336

NOISE

The noise of the AD8336 is dependent on the value of the VGA

gain. At maximum VGAIN, the dominant noise source is the

preamplifier, but it shifts to the VGA as VGAIN diminishes.

The input-referred noise at the highest VGA gain and a

preamplifier gain of 4×, with RFB1 = 100 Ω and RFB2 = 301 Ω, is

3 nV/Hz and is determined by the preamplifier and the gain

setting resistors. See Table 4 for the noise components for the

preamplifier.

Table 4. AD8336 Noise Components for Preamplifier Gain = 4×

Noise Component

Noise Voltage (nV/√Hz)

Op Amp (Gain = 4×)

2.6

RFB1 = 100 Ω

0.96

RFB2 = 301 Ω

0.55

VGA

0.77

Using the values listed in Table 4, the total noise of the AD8336

is slightly less than 3 nV/Hz, referred to the input. Although

the input noise referred to the VGA is 3.1 nV/Hz, the input-

referred noise at the preamplifier is 0.77 nV/Hz when divided

by the preamplifier gain of 4×.

At other than maximum gain, the noise of the VGA is

determined from the output noise. The noise in the center of

the gain range is about 150 nV/Hz. Because the gain of the

fixed-gain amplifier that is part of the VGA is 50×, the VGA

input-referred noise is approximately 3 nV/Hz, the same value

as the preamplifier and VGA combined. This is expected since

the input-referred noise is the same at the input of the attenuator at

maximum gain. However, the noise referred to the VGAI pin (the

preamplifier output) increases by the amount of attenuation

through the ladder network. The noise at any point along the

ladder network is primarily composed of the ladder resistance

noise, the noise of the input devices, and the feedback resistor

network noise. The ladder network and the input devices are

the largest noise sources.

At minimum gain, the output noise increases slightly to about

180 nV/Hz because of the finite structure of the X-AMP.

OFFSET VOLTAGE

Extensive cancellation circuitry included in the variable gain

amplifier section minimizes locally generated offset voltages.

However, when operated at very large values of gain, dc voltage

errors at the output can still result from small dc input voltages.

When configured for the nominal gain range of −14 dB to +46 dB,

the maximum gain is 200× and an offset of only 100 μV at the

input generates 20 mV at the output.

The primary source for dc offset errors is the preamplifier;

ac coupling between the PRAO and VGAI pins is the simplest

solution. In applications where dc coupling is essential, a comp-

ensating current can be injected at the INPN input (Pin 5) to

cancel preamplifier offset. The direction of the compensating

current depends on the polarity of the offset voltage.

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