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AD8335

Rev. 0 | Page 19 of 24

Gain Control

The gain control interface has two inputs, VGAIN (Pins VGNx)

and VSLP (Pins SLxx). The slope input is intended only as a

decoupling pin, and the only guaranteed gain slope is the

20 dB/V default. However, if a voltage is applied to the VSLP

inputs, the gain slope can be increased by reducing the slope

voltage. For example, if a voltage of 1.67 V is applied to Pins SLxx,

the gain slope changes to 30 dB/V. Use Equation 4 to calculate

the gain slope.

Slope

VSLP

dB/V

(4)

VGAIN varies the gain of the VGA through the interpolator by

selecting the appropriate input stages connected to the input

attenuator. The nominal VGAIN range for 20 dB/V is 0 V to 3 V,

with the best gain-linearity from approximately 0.5 V to 2.5 V,

where the error is typically less than ±0.2 dB. For VGAIN voltages

above 2.5 V and less than 0.5 V, the error increases (see Figure 4).

The value of the VGAIN voltage can be increased to that of the

supply voltage, without gain foldover.

Each channel has separate gain control pins that can be

connected to a common voltage-source such as found in most

ultrasound applications. For control of individual channels,

connect the appropriate gain control signal to each channel.

Output Stage

Duplicate output stages of the VGA provide an 8 dB (×2.5) gain

switch. The gain switch is intended to optimize the output noise

floor for either a 10-bit or 12-bit ADC. The VGA gain is 20 dB

(×10) in LO gain mode and 28 dB (×25) in HI gain mode. The

logic setting of the HILO (Pins HLxx) selects between output

amplifiers including the gain resistors and feedback buffers.

100 MHz bandwidth is maintained between the amplifiers by

changing the compensation capacitance as the gain switches gain

settings. Power consumption is the same for either level of gain.

In certain applications, power consumption can be reduced by

lowering the supply voltage as much as possible; however, the

output dynamic range is affected by the more limited swing. The

fully differential signal path of the AD8335 restores 6 dB of

dynamic range, and the common-mode level is maintained

automatically at half the supply voltage for maximum signal

swing. The differential signal has the added benefit of suppress-

ing the even order harmonics.

The output amplifier is designed to drive a nominal differential

load of 500 Ω or greater; the signal swing can be as large as

5 V p-p differential before clipping occurs. However, that distor-

tion increases before reaching the clipping level. Distortion is

shown in Figure 25 through Figure 34 for typical values of

1 V p-p or 2 V p-p (full-scale inputs for many ADCs). The

output is ac-coupled to a differential anti-alias filter driving a

differential ADC. Most modern ADCs have differential inputs

and achieve optimum performance when driven differentially.

For more information, see the Applications section.

VGA Noise

As with all X-AMPs, the output noise of the VGA is constant

with gain. This causes the input referred noise to increase as the

gain is decreased. This characteristic is desirable in receiver

applications where wide dynamic range input signals are com-

pressed with a fixed ceiling and noise floor into an ADC. The

VGA output noise is approximately 33 nV/√Hz in LO gain

mode and 2.5 times higher than this, 83 nV/√Hz, in HI gain

mode. As the gain increases, the noise of the preamplifier prevails

and, at the maximum VGA gain, the output noise is approxi-

mately 90 nV/√Hz and 225 nV/√Hz for LO and HI gain modes,

respectively.

The output SNR is determined by the noise floor and the largest

signal level, typically limited by the FS of the ADC. Modulation

noise, essentially the noise introduced by the gain control input,

can be troublesome. Normally one tends to look at the main

amplifier signal path for noise, but a VGA is really a multiplier

with the following function

REF

GAIN

OUT

(4)

where VREF (bias) and VGAIN (gain control interface) are both

noise contributors under certain conditions. It is therefore

important that the gain control signals be kept clean, especially

at higher gain control slopes.

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