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| 型號： | AD8264-EVALZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 21/40頁 |
| 文件大?。?/td> | 0K |
| 描述： | BOARD EVAL FOR AD8264 VGA |
| 標(biāo)準(zhǔn)包裝： | 1 |
| 每 IC 通道數(shù)： | 4 - 四 |
| 放大器類型： | 可變增益 |
| 輸出類型： | 差分 |
| 轉(zhuǎn)換速率： | 380 V/µs |
| -3db帶寬： | 235MHz |
| 工作溫度： | -40°C ~ 105°C |
| 電流供應(yīng)（主 IC）： | 99mA |
| 電壓 - 電源，單路/雙路(±)： | ±2.5 V ~ 5 V |
| 板類型： | 完全填充 |
| 已供物品： | 板 |
| 已用 IC / 零件： | AD8264 |
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AD8264
Rev. A | Page 28 of 40
THEORY OF OPERATION
OVERVIEW
The AD8264 is a dc-coupled quad channel VGA with a fixed
gain-of-2 (6 dB) preamplifier and a single-ended-to-differential
output amplifier with level shift capability that can be used as an
ADC driver. Figure 111 shows a representative block diagram of
a single channel; all four channels are identical. The supply can
operate from ±2.5 V to ±5 V. The primary application is as a
pulse processor for medical positron emission tomography
(PET) imaging; however, the part is useful for any dc-coupled
application that can benefit from variable gain.
The signal chain consists of three fundamental stages: the
preamplifier, the variable gain amplifier, and the differential
output buffer amplifier. The preamplifier has an internally fixed
gain-of-2 (6 dB). The VGA comprises an attenuator that
provides 0 dB to 24 dB of attenuation, followed by a fixed gain
18 dB (8×) amplifier. The single-ended VGA output is connected
directly to the noninverting input of the differential output
(post) amplifier, which has a differential fixed gain-of-2 (6 dB).
The gain range from the preamp input to the VGA output is
0 dB to 24 dB. The aggregate gain range from preamp input to
the differential postamplifier output is 6 dB to 30 dB.
The ideal gain equation for the gain from the single-ended
input to the output is
VGAIN = VGNHx VGNLO
(1)
ICPT
V
Gain
GAIN +
×
=
V
dB
20
(2)
The ideal value for ICPT, or the intercept, is defined at VGAIN = 0 V.
The ICPT for the VGA output and differential amplifier outputs
equals 12.1 dB and 18.1 dB, respectively. The actual intercept
varies with any additional gain or loss along the signal path.
The measured values are both approximately 0.2 dB low.
PREAMP
The preamplifier is a current feedback amplifier, designed to
drive the internal 100 gain setting resistors and the resistive
attenuator, which together result in a nominal load to the
preamplifier of about 113 . Normally, the negative preamp
input, IPNx, is not connected externally. The positive input
IPPx is the high impedance input of the current feedback amp.
Note that, at the largest supply voltage of ±5 V, the input signal
can become so large that the preamplifier output cannot deliver
the required current to drive the 113 load and, therefore, limits
at 6 V p-p. This means that the input limits at 3 V p-p.
The short-circuit input referred noise at maximum VGA gain is
about 2.3 nV/√Hz, and this accounts for all of the amplifiers and
gain setting resistors. When measuring the input referred noise
from the VGA output, the number is slightly lower at 2.1 nV/√Hz
because the noise of the postamplifier is not included in the
noise calculation.
VGA
The VGA has a voltage feedback architecture and uses analog
control to vary the gain. Its low gain range helps to maintain
low offset and is intended for gain trim applications. The offset
of the preamp and the VGA are trimmed; therefore, the maximum
input referred offset is <0.5 mV over temperature (see Figure 26).
Keeping the gain of each stage relatively low also allows the
bandwidth to stay high.
The gain of the VGA is adjusted using the fully differential
control inputs, GNHx and GNLO. The GNLO pin is internally
connected to all four channels and must be biased externally.
Under typical conditions, the GNLO pin is grounded. The gain
high control pins (GNHx) are independent for each channel.
The gain slope is nominally 20 dB/V. With GNLO connected to
ground, each GNHx input can have a voltage applied from VNEG
to VPOS without gain foldover.
To make use of the full gain range of the VGA, the nominal gain
control voltage needed at GNHx is ±0.65 V relative to the voltage
applied to GNLO. At the lowest supply voltage of ±2.5 V, the pin
GNLO should always be grounded. With increasing supply, the
common-mode range of the gain control interface increases.
This means that GNLO can be anywhere within ±1.2 V at
±3.3 V supplies and ±2.8 V at ±5 V supplies.
Table 5. Gain Control Input Range
SupplyVoltage (V)
GNLO Voltage Range (V)
V
GAIN Range (V)
±5
±2.8
±0.65
±3.3
±1.2
±0.65
±2.5
0
±0.65
For example, at ±3.3 V supplies, the outputs of a single-supply
unipolar DAC, such as the 10-bit, 4-channel AD5314, can be
used to drive the GNHx pins directly, in conjunction with using
Because the GNLO pin sources only about 1.2 A for the four
channels (~300 nA per channel, the same as for the GNHx pins), a
simple resistive divider is generally adequate to set the voltage at
the GNLO input.
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