OPA2822UG4 Datasheet, PDF (18/33 Pages) Texas Instruments – Dual, Wideband, Low-Noise Operational Amplifier

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


= 10 log^2

+
4
α
+
2^{}
e_n

1
2
+
1
α

/
n^{}
2
kTR_S
+
1
2
i_n nR_S
2



 ₍₁₎



where R_G = 1/2 n²R_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]
TABLE II. Noise Figure versus Gain with n = 2 Trans-
former.
TOTAL GAIN
(V/V)
4
5
6
7
8
9
10
LOG GAIN
(dB)
12.0
14.0
15.6
16.9
18.1
19.1
20.0
REQUIRED
AMPLIFIER GAIN
(α)
2
2.5
3
3.5
4
4.5
5
NOISE FIGURE
(dB)
11.2
10.4
9.9
9.5
9.1
8.9
8.6
+5V
+5V
R_S = 50Ω
V_I
R_G
100Ω
1:2
1/2
OPA2822
R_F
Noise
Figure
Defined
Here
V_{O = 2 RF}
V_I
R_G
R_G
100Ω
R_F
1/2
OPA2822
0.1µF
80Ω
1kΩ
V_I
100pF
V_O
500Ω
14-Bit
V_{CM ADC}
1µF
1kΩ
0.1µF
80Ω
V_I
100pF
–5V
FIGURE 10. Single-Ended to Differential High Dynamic Range ADC Driver.
18
OPA2822
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SBOS188E