AD8335 Datasheet, PDF (19/24 Pages) Analog Devices – Quad Low Noise, Low Cost Variable Gain Amplifier

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.
2.5 V× 20 dB/V
VSLP =
(4)
Slope
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
AD8335
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
V_OUT
= ^{VGAIN ×VIN}
V_REF
(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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