For the measured 2-tone, 3rd-order distortion for the ampli-
fier portion of the circuit of Figure 6, see 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 spurious until
25MHz, while the 2Vp-p is unmeasureable up to 40MHz
center frequency. 2-tone, 2nd-order intermodulation distor-
tion was unmeasureable for this circuit.
AC-COUPLED, DIFFERENTIAL INTERFACE WITH
GAIN
The interface circuit example presented in Figure 6 employs
two OPA687s, (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) located
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, VCM, 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 ADS5421. 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.
–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
VCM
20Ω
OPA847
100Ω
–5V
+5V
1.7pF
T1
39pF
39pF
850Ω
50Ω Source
1:2
IN
ADS5421
47pF
< 6dB
Noise
Figure
850Ω
IN
1.7pF
CM
VCM
+5V
100Ω
0.1µF
20Ω
OPA847
VCM
–5V
FIGURE 6. High Dynamic Range Interface Circuit with the OPA847 Set for a Gain of +8.5V/V.
ADS5421
12
SBAS237D
www.ti.com
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