V

V

ADC

REF

-

V

V

AMP

ADC

-

R

P

O

=R

GAIN=

2R ||Z

P

IN

2R +2R ||Z

O

P

IN

V

ADC FS

GAIN

V

THS770012

SLOS669C

– FEBRUARY 2010 – REVISED JANUARY 2012

www.ti.com

reasonable value, such as 49.9

(1)

The insertion of this resistor network also attenuates the amplifier output signal. The gain (or loss) can be

calculated by Equation 2:

(2)

the network can be calculated using Equation 3:

(3)

full-scale drive, and then for

–1dB (often times, the ADC drive is backed off from full-scale in applications, so

lower amplitudes may be acceptable). All voltages are in volts, resistors in

Ω (the nearest standard value should

be used), and gain as noted. Table 3 does not include the ADS5424 because no level shift is required with this

device.

GAIN

GAIN

ADC

(

Ω)

(

Ω)

(

Ω)

(V/V)

(dB)

ADS5485

2.5

3.1

5

1k

50

158.3

0.73

–2.71

2

4.10

3.65

ADS6149

2.5

1.5

0

NA

50

75.0

0.60

–4.44

2

3.33

2.97

ADS4149

2.5

0.95

0

NA

50

30.6

0.38

–8.40

2

5.26

4.69

ADS4149

0.95

2.5

NA

50

81.6

0.62

–4.15

2

3.23

2.88

(1)

THS770012 with

±2.5V supply.

this level, but the designer may want to further back off from full-scale for best performance and consider trading

reduced SNR performance for better SFDR performance.

The values calculated for the ADS6149 show reasonable design targets and should work with good performance.

Note the ADS6149 does not have buffered inputs, and the inputs have equivalent resistive impedance that varies

with sampling frequency. In order to account for the increased loss, half of this resistance should be used for the

The values calculated for the low input common-mode of the ADS4149 result in large attenuation of the amplifier

capability of the device. With a single +5V supply, the THS770012 is not suitable to drive this ADC in dc-coupled

applications unless the ADC input is backed off towards

–6dBFS. Another option is to operate the THS770012

with a split

±2.5V supply, and is shown in the last row of Table 3. For this situation, if the +2.5V is used as the

Supply

±2.5V section for more detail. Note that the ADS4149 does not have buffered inputs and the inputs have

equivalent resistive impedance that varies with sampling frequency. In order to account for the increased loss,

As with any design, testing is recommended to validate whether it meets the specific design goals.

22

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