OPA843 datasheet(17/32 Pages) TI2

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OPA2846

SBOS274C −JUNE 2003 − REVISED AUGUST 2008

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AC-COUPLED, SINGLE-TO-DIFFERENTIAL

ADC DRIVER

Where the signal path may be AC-coupled, a very

balanced, high SFDR dual op amp interface circuit can

easily be provided by the OPA2846. Figure 10 shows a

specific example of this application, where the input

single-to-differential conversion is provided by an input

transformer. Once the signal source is purely differential,

the circuit of Figure 10 provides low harmonic distortion

with a common-mode control path that does not interact

with the signal path gain. If the source is already

differential, such as at the output of a balanced mixer, the

input

transformer

could

replaced

blocking

capacitors.

In the example of Figure 10, the secondary of the

transformer is connected into the two inverting path gain

resistors (100

Ω). These resistors provide both an input

impedance match (assuming a 50

Ω source on the primary

of this 1:2 step-up transformer) and set the signal gain for

each amplifier along with the 500

Ω feedback resistors.

Although relatively high signal gain is provided by this

circuit (10 in this case), each amplifier is operating at a

relatively low noise gain (3.5V/V). This low-noise gain at

low frequencies gives high loop gain for distortion

suppression in the baseband. External compensation

capacitors (18pF and 2.1pF) are included to hold the

frequency response flat, as described in the Low-Gain

Compensation For Improved SFDR section of this data

sheet. The common-mode operating voltage is fed into

each amplifier’s noninverting input. Since these are equal,

and will appear at each inverting input as well, no DC

current is produced through the transformer secondary

due to this common-mode operating voltage. Since no

current flows due to VCM, the output will operate at VCM as

well. This is one of the few common-mode operating point

control techniques that requires no current to flow. This

makes the common-mode control aspect of this circuit

essentially non-interactive with the signal path. To provide

a 2VPP differential signal operating at a 2.5V output

common-mode requires a 2.0V to 3.0V output swing on

each output. Tested performance over frequency for the

circuit of Figure 10 is shown in Figure 11.

Single−to−Differential

Gain of 10

Power−supply decoupling

not shown.

CM +10VI

1/2

OPA2846

2.5V

+5V

500

Ω

500

Ω

µF

−

V+

14−Bit

10MSPS

ADS850

2.1pF

1/2

OPA2846

500

Ω

−5V

18pF

1000pF

18pF

100

Ω

100

Ω

500

Ω

2.1pF

Ω

1:2

Figure 10. AC-Coupled, Single-to-Differential High SFDR ADC Driver

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