Datasheet
OPA2822
18
SBOS188E
www.ti.com
HIGH DYNAMIC RANGE ADC DRIVER
Numerous circuit approaches exist to provide the last stage
of amplification before the ADC in high-performance applica-
tions. For very high dynamic range applications where the
signal channel can be AC-coupled, the circuit shown in
Figure 10 provides exceptional performance. Most very high
performance ADCs > 12-bit performance require differential
inputs to achieve the dynamic range. The circuit of Figure 10
converts a single-ended source to differential via a 1:2 turns
ratio transformer, which then drives the inverting gain setting
resistors (R
G
). These resistors are fixed at 100Ω to provide
input matching to a 50Ω source on the transformer primary
side. The gain can then be adjusted by setting the feedback
resistor values. For best performance, this circuit operates
with a ground centered output on ±5V supplies, although a
+12V supply can also provide excellent results. Since most
high-performance converters operate on a single +5V sup-
ply, the output is level shifted through an AC blocking
capacitor to the common-mode input voltage (V
CM
) for the
converter input, and then low-pass filtered prior to the input
of the converter. This circuit is intended for inputs from 10kHz
to 10MHz, so the output high-pass corner is set to 1.6kHz,
while the low-pass cutoff is set to 20MHz. These are example
cutoff frequencies; the actual filtering requirements would be
set by the specific application.
The 1:2 turns ratio transformer also provides an improvement
in input referred noise figure. Equation 1 shows the Noise
Figure (NF) calculation for this circuit, where R
G
has been
constrained to provide an input match to R
S
(through the
transformer) and then R
F
is set to get the desired overall
gain. With these constraints (and 0Ω on the noninverting
inputs), the noise figure equation simplifies considerably.
NF
eninR
kTR
nn
S
S
=++
+
+
(
)
10 2
4
2
1
2
11
2
2
2
log
/
α
α
(1)
where R
G
= 1/2 n
2
R
S
n = Transformer Turns Ratio
α =R
F
/R
G
e
n
= Op Amp Input Voltage Noise
i
n
= Inverting Input Current Noise
kT = 4E – 21J[T = 290°K]
Gain (dB) = 20 log[nα]
REQUIRED
TOTAL GAIN LOG GAIN AMPLIFIER GAIN NOISE FIGURE
(V/V) (dB) (α)(dB)
4 12.0 2 11.2
5 14.0 2.5 10.4
6 15.6 3 9.9
7 16.9 3.5 9.5
8 18.1 4 9.1
9 19.1 4.5 8.9
10 20.0 5 8.6
TABLE II. Noise Figure versus Gain with n = 2 Trans-
former.
FIGURE 10. Single-Ended to Differential High Dynamic Range ADC Driver.
1/2
OPA2822
+5V
1:2
+5V
–5V
Noise
Figure
Defined
Here
1/2
OPA2822
V
O
V
I
V
O
V
I
R
F
R
G
V
I
R
F
R
F
R
G
100Ω
R
G
100Ω
V
CM
1kΩ
1kΩ
14-Bit
ADC
0.1µF
80Ω
80Ω
100pF
100pF
1µF
0.1µF
500Ω
V
I
R
S
= 50Ω
= 2