ADS5422 Datasheet, PDF (11/19 Pages) Texas Instruments – 14-Bit, 62MSPS Sampling ANALOG-TO-DIGITAL CONVERTER

AC-COUPLED, DIFFERENTIAL INTERFACE
WITH GAIN
The interface circuit example presented in Figure 6 employs
two OPA847s (decompensated voltage-feedback op amps),
optimized for gains of 12V/V or higher. Implementing a new
compensation technique allows the OPA847s to operate with
a reduced signal gain of 8.5V/V, while maintaining the high
loop gain and the associated excellent distortion perfor-
mance offered by the decompensated architecture. For a
detailed discussion on this circuit and the compensation
scheme, refer to the OPA847 data sheet (SBOS251) avail-
able at www.ti.com. Input transformer, T1, converts the
single-ended input signal to a differential signal required at
the inverting inputs of the amplifier, which are tuned to
provide a 50Ω impedance match to an assumed 50Ω source.
To achieve the 50Ω input match at the primary of the 1:2
transformer, the secondary must see a 200Ω load imped-
ance. Both amplifiers are configured for the inverting mode
resulting in close gain and phase matching of the differential
signal. This technique, along with a highly symmetrical lay-
out, is instrumental in achieving a substantial reduction of the
2nd-harmonic, while retaining excellent 3rd-order perfor-
mance. A common-mode voltage, V_CM, is applied to the
noninverting inputs of the OPA847. Additional series 20Ω
resistors isolate the output of the op amps from the capaci-
tive load presented by the 40pF capacitors and the input
capacitance of the ADS5422. This 20Ω/47pF combination
sets a pole at approximately 85MHz and rolls off some of the
wideband noise resulting in a reduction of the noise floor.
The measured 2-tone, 3rd-order distortion for the amplifier
portion of the circuit of Figure 6 is shown in Figure 7. The
upper curve is for a total 2-tone envelope of 4Vp-p, requiring
two tones, each 2Vp-p across the OPA847 outputs. The
lower curve is for a 2Vp-p envelope resulting in a 1Vp-p
amplitude per tone. The basic measurement dynamic range
for the two close-in spurious tones is approximately 85dBc.
The 4Vp-p test does not show measurable 3rd-order spuri-
ous until 25MHz, while the 2Vp-p is unmeasurable up to
40MHz center frequency. 2-tone, 2nd-order intermodulation
distortion was unmeasurable for this circuit.
–60
–65
4Vp-p
–70
–75
2Vp-p
–80
–85
0
5 10 15 20 25 30 35 40 45 50
Center Frequency (MHz)
FIGURE 7. Measured 2-Tone, 3rd-Order Distortion for a
Differential ADC Driver.
+5V
V_CM
20Ω
100Ω
OPA847
T₁
50Ω Source 1:2
39pF
–5V
1.7pF
850Ω
< 6dB
Noise
Figure
39pF
850Ω
1.7pF
100Ω
+5V
20Ω
OPA847
V_CM
–5V
+5V
47pF
IN
ADS5422
IN
CM
V_CM
0.1µF
FIGURE 6. High Dynamic Range Interface Circuit with the OPA847 Set for a Gain of +8.5V/V.
ADS5422
11
SBAS250C
www.ti.com