OPA2690 datasheet(18/33 Pages) TI

18 / 33 page

OPA2822

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 (RG). These resistors are fixed at 100Ω to provide

input matching to a 50

Ω 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 (VCM) 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 RG has been

constrained to provide an input match to RS (through the

transformer) and then RF is set to get the desired overall

gain. With these constraints (and 0

Ω on the noninverting

inputs), the noise figure equation simplifies considerably.

i nR

kTR

















(

)













log

(1)

where RG = 1/2 n2RS

n = Transformer Turns Ratio

α =R

F/RG

en = Op Amp Input Voltage Noise

in = Inverting Input Current Noise

kT = 4E – 21J[T = 290

°K]

Gain (dB) = 20 log[n

α]

REQUIRED

TOTAL GAIN

LOG GAIN

AMPLIFIER GAIN

NOISE FIGURE

(V/V)

(dB)

(

α)(dB)

12.0

11.2

14.0

2.5

10.4

15.6

9.9

16.9

3.5

9.5

18.1

9.1

19.1

4.5

8.9

20.0

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

100

Ω

100

Ω

14-Bit

ADC

0.1

µF

Ω

100pF

µF

0.1

µF

500

Ω

S = 50Ω

= 2

OPA2690 Datasheet(PDF) 18 Page - Texas Instruments

OPA2690 Datasheet(HTML) 18 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

Datasheet Download

Link URL