Over-Range Analog Input Recovery Error

Analog Input Amplitude − dBFS

−4

−3

−2

−1

0

1

2

3

4

−1

0

1

2

3

4

5

6

G055

Test fIN = Fclkmax/4 Sinusoid

Expected In-Range Code is 2048 (Mid-Scale)

Input Amplitude Values >0 dBFS are

Over-Ranging the ADC

Error After Positive Over-Range

Error After Negative Over-Range

ADS5463

ADS54RF63

SLAS515E – NOVEMBER 2006 – REVISED JULY 2009 ................................................................................................................................................... www.ti.com

For applications requiring dc-coupling with the signal source, a differential input/differential output amplifier like

the THS4509 (see Figure 51) provides good harmonic performance and low noise over a wide range of

frequencies. Notice that VREF is used for the common mode with the ADS5463/ADS54RF63 and

ADS5444/5440, whereas VCM must be used with the ADS5474.

In this configuration, the THS4509 amplifier circuit provides 10 dB of gain, converts the single-ended input to

differential, and sets the proper input common-mode voltage to the ADS5463/ADS54RF63 by using the VREF

pin from the ADC. The 50-

Ω resistors and 18-pF capacitor between the THS4509 outputs and

ADS5463/ADS54RF63 inputs (along with the input capacitance of the ADC) limit the bandwidth of the signal to

about 70 MHz (–3 dB). Input termination is accomplished via the 78.9-

Ω resistor and 0.22-

µF capacitor to

ground, in conjunction with the input impedance of the amplifier circuit. A 0.22-

µF capacitor and 49.9-Ω resistor

are inserted to ground across the 78.9-

Ω resistor and 0.22-

µF capacitor on the alternate input to balance the

circuit. Gain is a function of the source impedance, termination, and 348-

Ω feedback resistor. See the THS4509

data sheet for further component values to set proper 50-

Ω termination for other common gains. Because the

ADS5463/ADS54RF63 recommended input common-mode voltage is 2.4 V, the THS4509 is operated from a

single power supply input with Vs+ = 5 V and Vs– = 0 V (ground). This maintains maximum headroom on the

internal transistors of the THS4509.

An over-range condition occurs if the analog input voltage exceeds the full-scale range of the converter (0dBFS,

nominally 2.2 Vpp). To test recovery from an over-range, the ADC analog input is injected with a sinusoidal input

frequency exactly at CLKIN/4 (a four-point sinusoid). The four sample points of each period theoretically occur at

the top, mid-scale, bottom and mid-scale of the sinusoid. Once the amplitude exceeds 0dBFS, the top and

bottom of the sinusoidal input becomes out of range, while the mid-scale points are always in-range and

therefore measureable. The graph in Figure 52 indicates the amount of error from the expected mid-scale value

of 2048 that occurs after negative over-range (bottom of sinusoid went out of range) and positive over-range (top

of sinusoid went out of range). Due to the four point sinusoid, this equates to the amount of error in a valid

sample 1 clock cycle after an over-range occurs as a function of amplitude. The errors generally increase as a

function of input over-range amplitude, though non-monotonically.

R

R

R

OVER-RANGE RECOVERY ERROR

vs

ANALOG INPUT AMPLITUDE

Figure 52.

28

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