ada4922-1 Analog Devices, Inc., ada4922-1 Datasheet - Page 16

• Manufacturer Part Number: ada4922-1
• Description: High Voltage, Differential 18-bit Adc Driver
• Manufacturer: Analog Devices, Inc.

ADA4922-1
APPLICATIONS
The ADA4922-1 is a fixed-gain, single-ended-to-differential
voltage amplifier, optimized for driving high resolution ADCs
in high voltage applications. There are no gain adjustments
available to the user.
ADA4922-1 DIFFERENTIAL OUTPUT NOISE MODEL
The principal noise sources in a typical ADA4922-1 application
circuit are shown in Figure 49.
V
Using the traditional approach, a noise source is applied in
series with one of the inputs of each op amp to model input-
referred voltage noise. The input current noise that matters the
most is present at the input pin. The output voltage noise due to
this noise current depends on the source resistance feeding the
input, as well as the downstream gain in the amplifier. Resistor
noise is modeled by placing a noise voltage source in series with
a noiseless resistor. R
the same noise voltage density.
At room temperature,
The noise at OUT+ is due to the input-referred current and
voltage noise sources of the noninverting amplifier and the
noise of the source resistance, all reflected to the output with a
noise gain of 1, and is equal to:
Voltage Noise @ OUT+: V
where R
the source resistance noise.
The noise at OUT− originates from a number of sources:
Voltage Noise @ OUT− due to V
Voltage Noise @ OUT− due to I
Voltage Noise @ OUT− due to R
Voltage Noise @ OUT− due to V
V
nRs
nRg
R
s
=
V
I
V
n1
n1
S
nRf
is the source resistance feeding the input, and V
Figure 49. ADA4922-1 Differential Output Noise Model
=
4
kT
(
600
f
and R
Ω
n1
)
g
+ R
≈
V
are both 600 Ω and therefore have
nRg
3.2
nRg
n1
REF
S
S
(I
n1
:
:
:
nV/
:
n1
R
V
V
R
g
V
) + V
nRs
nRg
V
S
n2
n1
( )
I
⎛ −
⎜
⎜
⎝
⎛ −
⎜
⎜
⎝
⎛ −
⎜
⎜
⎝
n
Hz
1
nRs
R
R
R
⎛ −
⎜
⎜
⎝
R
R
R
g
g
g
V
f
f
R
f
nRf
R
⎞
⎟
⎟
⎠
⎞
⎟
⎟
⎠
g
⎞
⎟
⎟
⎠
f
=
=
=
⎞
⎟
⎟
⎠
−
−
−
R
=
f
V
V
V
−
nRs
n1
nRg
R
S
( )
I
nRs
n
OUT–
OUT+
1
is
Rev. 0 | Page 16 of 20
(2)
(3)
(4)
(5)
(6)
(7)
Voltage Noise @ OUT− due to V
Voltage Noise @ OUT− due toV
When looking at OUT− by itself, the contributing noise sources
are uncorrelated, and therefore, the total output noise is
calculated as the root-sum-square (rss) of the individual
contributors. When looking at the differential output noise, the
noise contributors are uncorrelated except for three, V
and V
can be seen from the previous results that the output noise due
to V
and at OUT− with a gain of −1. This produces a gain of 2 for each
of these three sources at the differential output.
The total differential output noise density is calculated as
V
where V
noise of each amplifier is the same.
The output noise due to the amplifier alone is calculated by
setting R
V
Clearly, the output noise is not balanced between the outputs,
but this is not an issue in most applications.
USING THE REF PIN
The REF pin sets the output baseline in the inverting path and
is used as a reference for the input signal. In most applications,
the REF pin is set to the input signal midswing level, which in
many cases is also midsupply. For bipolar signals and power
supplies, REF is generally set to ground. In single-supply
applications, setting REF to the input signal midswing level
provides optimal output dynamic range performance with
minimum differential offset. Note that the REF input only
affects the inverting signal path, or OUT−.
Most applications require a differential output signal with the
same dc common-mode level on each output. It is possible for
the signal measured across OUT+ and OUT− to have a common-
mode voltage that is of the desired level but has different dc
levels at both outputs. Typically, this situation is avoided,
because it wastes the amplifier’s output dynamic range.
on, dm
on, dm
(
2
(
n1
V
, R
=
n
nRs
= 12 nV/√Hz
+
, which are common noise sources for both outputs. It
n1
S
S
(I
R
and V
= V
s
n1
(1.4
), and V
n2
pA/
nRs
≡ V
equal to zero. In this case:
nRs
n
Hz
= 3.9 nV/√Hz; the input referred voltage
each appear at OUT+ with a gain of +1
)
+
V
nRs
n2
)
nRf
) (
:
2
: V
V
+
n2
2
nRF
⎛
⎜
⎜
⎝
3.2
1
+
nV/
R
R
g
f
⎞
⎟
⎟
⎠
Hz
=
2
V
)
2
n
+
2
4
n1
V
, R
n
2
S
(I
(10)
(11)
n1
(8)
(9)
),