THS3215 Datasheet, PDF (35/71 Pages) Texas Instruments – THS3215 650-MHz, Differential to Single-Ended DAC Output Amplifier

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THS3215
SBOS780A – MARCH 2016 – REVISED APRIL 2016
Feature Description (continued)
The buffer provides a very wideband, low output-impedance when used to drive VREF, pin 14 (see Figure 51).
To provide this low broadband impedance, the closed-loop midscale (dc) reference buffer offers a very
broadband SSBW, but only a modest large-signal bandwidth (LSBW); see Figure 49. This path is not normally
intended to inject a wideband signal, but can be used for lower-amplitude signals. Driving the buffer output into
the VREF pin allows a wideband small-signal term to be added into the D2S along with the signal from the
differential inputs.
The midscale (or dc) reference buffer injects an offset voltage to the output offset of the D2S when it drives the
VREF pin. The offset has very low drift, but consider the effect of the input bias current times the dc source
impedance at VMID_IN (pin 1). When used as a default midsupply reference for single-supply operation, the
input to this buffer is just the average of the total power supplies though a 25-kΩ source impedance. Add an
external capacitor to filter the supply and the 50-kΩ internal resistors. A 1-µF capacitor on VMID_IN adds a 6-Hz
pole to the noise sources. If lower noise at lower frequencies is required, implement a midscale divider with
external, lower-valued resistors in parallel with the internal 50-kΩ values.
If the midscale buffer drives the VREF pin, the buffer noise is added to Equation 9 and Equation 10. The
midscale buffer 4.4-nV/√Hz voltage noise is amplified by 0 dB, and adds (RMS) a negligible impact to the total
D2S output noise. The biggest impact comes when the internal default 50-kΩ dividers are used. Be sure to
decouple VMID_IN with at least a 1-µF capacitor in the application to reduce the noise contribution through this
path. Figure 80 shows the simulation circuit with the 1-µF capacitor installed. Figure 81 shows the simulated
output noise for the midscale buffer using the internal 50-kΩ divider with and without the 1-µF capacitor on
VMID_IN.
+VCC2
16
50 k
VMID_IN
1
1 …F
50 k
x1
15 VMID_OUT
5
-VCC2
Figure 80. Midscale Buffer Noise Model
1000
With 1PF capacitor
Without 1PF capacitor
100
10
1
10
100
1k
10k
100k
1M
Frequency (Hz)
D501
Figure 81. Buffer-Output Noise Comparison With and Without the 1-µF Bypass Capacitor on VMID_IN
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