O P A820

+5V

+V

S

DIS

100Ω

R

G

402Ω

806Ω

806Ω

57.6Ω

0.01µF

6.8µF

0.1µF

0.01µF

V

I

50Ω Source

R

F

402Ω

V

V

O

OPA820

+5V

+V

S

DIS

100Ω

R

G

402Ω

806Ω

806Ω

0.01µF

0.01µF

V

I

R

F

402Ω

V

V

O

6.8µF

0.1µF

OPA820-HT

SBOS587

– DECEMBER 2011

www.ti.com

signal range. The input impedance matching resistor (57.6

Ω) used in Figure 4 is adjusted to give a 50-Ω input

AC-coupled, giving the circuit a DC gain of +1. This puts the input DC bias voltage (2.5 V) on the output as well.

On a single 5-V supply, the output voltage can swing to within 1.3 V of either supply pin while delivering more

than 80-mA output current giving 2.4-V output swing into 100

Ω (5.6-dBm maximum at the matched load).

Figure 5 shows the AC-coupled, single 5-V supply, gain of

−1 V/V circuit configuration used as a basis for the

5 V only Typical Characteristic curves. In this case, the midpoint DC bias on the noninverting input is also

decoupled with an additional 0.01-

μF decoupling capacitor. This reduces the source impedance at higher

frequencies for the noninverting input bias current noise. This 2.5-V bias on the noninverting input pin appears on

The single-supply test circuits of Figure 4 and Figure 4 show 5-V operation. These same circuits can be used

over a singlesupply range of 5 V to 12 V. Operating on a single 12-V supply, with the Absolute Maximum Supply

voltage specification of 13 V, gives adequate design margin for the typical

±5% supply tolerance.

Figure 4. AC-Coupled, G = +2 V/V, Single-Supply Specifications and Test Circuit

Figure 5. AC-Coupled, G =

−1 V/V, Single-Supply Specifications and Test Circuit

Buffering High-Performance ADCs

To achieve full performance from a high dynamic range ADC, considerable care must be exercised in the design

of the input amplifier interface circuit. The example circuit on the front page shows a typical AC-coupled interface

to a very high dynamic range converter. This AC-coupled example allows the OPA820 to be operated using a

blocking capacitor to a midscale reference level, which is created by a well-decoupled resistive divider off the

converter

’s internal reference voltages. To have a negligible effect (1 dB) on the rated spurious-free dynamic

range (SFDR) of the converter, the amplifier

’s SFDR should be at least 18 dB greater than the converter. The

18

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